Refactors walsenders out of timeline.rs to makes it less convoluted into
separate WalSenders with its own lock, but otherwise having the same structure.
Tracking of in-memory remote_consistent_lsn is also moved there as it is mainly
received from pageserver.
State of walsender (feedback) is also restructured to be cleaner; now it is
either PageserverFeedback or StandbyFeedback(StandbyReply, HotStandbyFeedback),
but not both.