Decouple channel management from host session management

[eddyashton]

This PR includes the final pieces from #2775:

• Node-to-node sessions on the host are not managed by the enclave. Instead they are created on-demand by the host, and should be cullable on idleness independently of the enclave's channel key management
• Key exchange protocol is rewritten to clarify what is serialised in each message, with a few functional tweaks. The core idea is that we more often obey a new init message, regardless of what we were doing, and try to proceed with that exchange
• ChannelManager implementation is separated from interface, and only the manager (not the internal Channels) are exposed by the API/used by the tests

[achamayou]

I've not managed to reproduce exactly what's happening in CI, but I've hit a run where one of the new nodes essentially fails to catch up, seemingly because it refuses to establish a channel successfully with the current primary (one of the old nodes).

It then stays stuck in WAITING_FOR_FINAL, seemingly forever.

2021-08-10T16:58:53.252032Z -0.006 0   [info ] ../src/node/node_state.h:657         | Node n[03b2170c2ad208e6d3a895e37d45c539cd89e0e644549328997e9aaf0aa98187] is waiting for votes of members to be trusted
2021-08-10T16:58:57.301365Z -0.073 0   [info ] ../src/consensus/aft/raft.h:3352     | Added raft node n[7914f7a2941bbf6c0ebcf8c45b165992687c641c5625888d962817e1bd717fc1] (127.168.43.206:40165)
2021-08-10T16:58:57.301419Z -0.073 0   [info ] ../src/consensus/aft/raft.h:3352     | Added raft node n[7a02f4becd3615bf6f92f0c5e0c6a5137cd53002a91210e5301c71c3fc4f5850] (127.109.68.148:37233)
2021-08-10T16:58:57.301442Z -0.073 0   [info ] ../src/consensus/aft/raft.h:3352     | Added raft node n[a5085d99316d81c177c5fab55986dbb7dfc5c7113b9e8dafe84bd52f98cd0ea2] (127.20.34.178:38167)
2021-08-10T16:58:57.301463Z -0.073 0   [info ] ../src/consensus/aft/raft.h:3352     | Added raft node n[d3e5d133b737de6c691c7d79864a5052ab5c483f1335e7ca2f68f4e6e246f85f] (127.114.210.171:42657)
2021-08-10T16:58:57.301515Z        100 [info ] ../src/host/ledger.h:822             | Setting last known/commit index to 131
2021-08-10T16:58:57.301535Z -0.073 0   [info ] ../src/consensus/aft/raft.h:2866     | Becoming follower n[03b2170c2ad208e6d3a895e37d45c539cd89e0e644549328997e9aaf0aa98187]: 2
2021-08-10T16:58:57.307910Z -0.079 0   [info ] ../src/node/node_state.h:626         | Joiner successfully resumed from snapshot at seqno 131 and view 2
2021-08-10T16:58:57.312170Z -0.084 0   [info ] ../src/node/node_state.h:1591        | Network TLS connections now accepted
2021-08-10T16:58:57.313251Z -0.085 0   [info ] ../src/node/node_state.h:647         | Node has now joined the network as node n[03b2170c2ad208e6d3a895e37d45c539cd89e0e644549328997e9aaf0aa98187]: all domains
2021-08-10T16:58:57.331681Z -0.009 0   [info ] ../src/node/channels.h:902           | Resetting channel with n[a5085d99316d81c177c5fab55986dbb7dfc5c7113b9e8dafe84bd52f98cd0ea2]
2021-08-10T16:58:57.402321Z -0.004 0   [info ] ../src/node/channels.h:902           | Resetting channel with n[a5085d99316d81c177c5fab55986dbb7dfc5c7113b9e8dafe84bd52f98cd0ea2]
2021-08-10T16:58:57.408753Z -0.011 0   [fail ] ../src/node/channels.h:531           | <- n[a5085d99316d81c177c5fab55986dbb7dfc5c7113b9e8dafe84bd52f98cd0ea2] (WAITING_FOR_FINAL): Peer certificate verification failed
2021-08-10T16:58:57.408818Z -0.001 0   [info ] ../src/node/channels.h:814           | Node channel with n[a5085d99316d81c177c5fab55986dbb7dfc5c7113b9e8dafe84bd52f98cd0ea2] cannot receive authenticated message: not established, status=WAITING_FOR_FINAL
2021-08-10T16:58:57.409902Z -0.002 0   [info ] ../src/consensus/aft/raft.h:867      | Dropped invalid message from n[a5085d99316d81c177c5fab55986dbb7dfc5c7113b9e8dafe84bd52f98cd0ea2]
2021-08-10T16:58:57.492951Z -0.001 0   [info ] ../src/node/channels.h:814           | Node channel with n[a5085d99316d81c177c5fab55986dbb7dfc5c7113b9e8dafe84bd52f98cd0ea2] cannot receive authenticated message: not established, status=WAITING_FOR_FINAL
2021-08-10T16:58:57.492992Z -0.001 0   [info ] ../src/consensus/aft/raft.h:867      | Dropped invalid message from n[a5085d99316d81c177c5fab55986dbb7dfc5c7113b9e8dafe84bd52f98cd0ea2]
2021-08-10T16:58:57.661553Z -0.001 0   [info ] ../src/node/channels.h:814           | Node channel with n[a5085d99316d81c177c5fab55986dbb7dfc5c7113b9e8dafe84bd52f98cd0ea2] cannot receive authenticated message: not established, status=WAITING_FOR_FINAL
2021-08-10T16:58:57.661602Z -0.001 0   [info ] ../src/consensus/aft/raft.h:867      | Dropped invalid message from n[a5085d99316d81c177c5fab55986dbb7dfc5c7113b9e8dafe84bd52f98cd0ea2]
2021-08-10T16:58:57.801621Z -0.056 0   [info ] ../src/node/channels.h:814           | Node channel with n[a5085d99316d81c177c5fab55986dbb7dfc5c7113b9e8dafe84bd52f98cd0ea2] cannot receive authenticated message: not established, status=WAITING_FOR_FINAL
2021-08-10T16:58:57.801679Z -0.056 0   [info ] ../src/consensus/aft/raft.h:867      | Dropped invalid message from n[a5085d99316d81c177c5fab55986dbb7dfc5c7113b9e8dafe84bd52f98cd0ea2]
2021-08-10T16:58:57.917641Z -0.001 0   [info ] ../src/node/channels.h:814           | Node channel with n[a5085d99316d81c177c5fab55986dbb7dfc5c7113b9e8dafe84bd52f98cd0ea2] cannot receive authenticated message: not established, status=WAITING_FOR_FINAL