Extract safekeeper per timeline state from safekeeper.rs

safekeeper.rs is mostly about consensus, but state is wider. Also form
SafekeeperState which encapsulates persistent part + in memory layer with API
for atomic updates.

Moves remote_consistent_lsn back to SafekeeperMemState, fixes its absense from
memory dump.

Also renames SafekeeperState to TimelinePersistentState, as TimelineMemState and
TimelinePersistent state are created.

safekeeper/src/json_ctrl.rs

@@ -21,7 +21,8 @@ use crate::safekeeper::{AcceptorProposerMessage, AppendResponse, ServerInfo};
 use crate::safekeeper::{
     AppendRequest, AppendRequestHeader, ProposerAcceptorMessage, ProposerElected,
 };
-use crate::safekeeper::{SafeKeeperState, Term, TermHistory, TermLsn};
+use crate::safekeeper::{Term, TermHistory, TermLsn};
+use crate::state::TimelinePersistentState;
 use crate::timeline::Timeline;
 use crate::GlobalTimelines;
 use postgres_backend::PostgresBackend;
@@ -56,7 +57,7 @@ pub struct AppendLogicalMessage {
 #[derive(Debug, Serialize)]
 struct AppendResult {
     // safekeeper state after append
-    state: SafeKeeperState,
+    state: TimelinePersistentState,
     // info about new record in the WAL
     inserted_wal: InsertedWAL,
 }