[#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

replication/logic/pdu/pdu.proto

@@ -9,7 +9,6 @@ service Replication {
   rpc DestroySnapshots(DestroySnapshotsReq) returns (DestroySnapshotsRes);
   rpc ReplicationCursor(ReplicationCursorReq) returns (ReplicationCursorRes);
   rpc SendCompleted(SendCompletedReq) returns (SendCompletedRes);
-  rpc HintMostRecentCommonAncestor(HintMostRecentCommonAncestorReq) returns (HintMostRecentCommonAncestorRes);
   // for Send and Recv, see package rpc
 }
 
@@ -126,19 +125,3 @@ message PingRes {
   // Echo must be PingReq.Message
   string Echo = 1;
 }
-
-message HintMostRecentCommonAncestorReq {
-  string Filesystem = 1;
-
-  // A copy of the FilesystemVersion on the sending side that the replication
-  // algorithm identified as a shared most recent common version between sending
-  // and receiving side.
-  //
-  // If nil, this is an indication that the replication algorithm could not
-  // find a common ancestor between the two sides.
-  // NOTE: nilness does not mean that replication never happened - there could
-  // as well be a replication conflict. thus, dont' jump to conclusions too
-  // rapidly here.
-  FilesystemVersion SenderVersion = 2;
-}
-message HintMostRecentCommonAncestorRes {}