accumulate gossip data until it can be sent

[rade]

Introduce an intermediary - GossipSender - between GossipChannel and
Connection. This accumulates gossip data (now represented by the new
GossipData interface) from the former until it can be passed onto the
latter, allowing the former to proceed when the latter may be blocked
on i/o.

This

1. prevents deadlocks that arise from cycles in the communication
   topology

2. improves performance by allowing GossipChannel and its calling code
   to proceed when connection i/o is blocked

3. improves performance by accumulating GossipData - the accumulated
   data often is considerably more compact than the sum of all the
   accumulated bits, and it is transmitted in a single communication
   event.

In order to support accumulation, the GossipData interface has a Merge
method. Furthermore, encoding is deferred until the data can actually
be sent, since accumulation would be hard/impossible to encoded
data. This required changes on some interfaces, most notably Gossiper.

The implementation of GossipData for topology gossip is
TopologyGossipData. It carries a reference to Peers and a set of
PeerNames, indexing into Peers and referencing the Peer entries which
have changed. Previously updates were represented as a list of Peer
entries. The indirection via PeerNames allows Encode to omit entries
which have been removed. The change in representation required changes
to the signature of some methods on Peers.

Note that all the above only applies to the "periodic gossip" portion
of the Gossiper API, which topology gossip in fact uses for all
gossip. GossipUnicast and GossipBroadcast are unchanged, but could
conceivably receive the same treatment in the future.

Fixes #445.