CandidateSet address books are not disjoint

[teor2345]

Description

The CandidateSet address books: disconnected, gossiped, and failed, are not disjoint. But the module documentation says they should be.

Analysis

Peers are not removed from the other sets when they are added to the failed set in report_failed. So the sets can not possibly be disjoint.

Suggested Solution

Core changes:

1. add a PeerState enum field to the MetaAddr
2. implement the time-based state as a MetaAddr method, rather than an enum variant
3. delete the disconnected, gossiped, and failed peer sets, and replace them with lookups into the single peer set

Optional improvements:
4. delete unused code

After these changes, there is only one copy of each peer, and it can only be in one state.