[#316] endpoint / replication protocol: more robust step-holds and replication cursor management

- drop HintMostRecentCommonAncestor rpc call
    - it is wrong to put faith into the active side of the replication to always make that call
      (we might not trust it, ref pull setup)
- clean up step holds + step bookmarks + replication cursor bookmarks on
  send RPC instead
    - this makes it symmetric with Receive RPC
- use a cache (endpoint.sendAbstractionsCache) to avoid the cost of
  listing the on-disk endpoint abstractions state on every step

The "create" methods for endpoint abstractions (CreateReplicationCursor, HoldStep) are now fully
idempotent and return an Abstraction.

Notes about endpoint.sendAbstractionsCache:
- fills lazily from disk state on first `Get` operation
- fill from disk is generally only attempted once
    - unless the `ListAbstractions` fails, in which case the fill from
      disk is retried on next `Get` (the current `Get` will observe a
      subset of the actual on-disk abstractions)
    - the `Invalidate` method is called
- it is a global (zrepl process-wide) cache

fixes #316

rpc/versionhandshake/versionhandshake.go

@@ -152,7 +152,7 @@ func (m *HandshakeMessage) DecodeReader(r io.Reader, maxLen int) error {
 
 func DoHandshakeCurrentVersion(conn net.Conn, deadline time.Time) *HandshakeError {
 	// current protocol version is hardcoded here
-	return DoHandshakeVersion(conn, deadline, 3)
+	return DoHandshakeVersion(conn, deadline, 4)
 }
 
 const HandshakeMessageMaxLen = 16 * 4096