Instead of passing a mutable outbox to all sub-systems, have all
sub-systems implement `Iterator`.

This was not used.

Instead of having each manager module have its own events, we use
a single event namespace. This simplifies the client event mapper,
which can be removed in a subsequent commit.

Instead of relying on the return value when receiving a `verack`, we
rely on the `PeerNegotiated` event, internalizing more of the logic.

In passing, we remove the `Socket` abstraction, which wasn't used.

Further reduce logic inside `fsm.rs`.

Since these are used in logs, they should be consistent with the other
logs.