[#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/rpc_client.go

@@ -142,13 +142,6 @@ func (c *Client) SendCompleted(ctx context.Context, in *pdu.SendCompletedReq) (*
 	return c.controlClient.SendCompleted(ctx, in)
 }
 
-func (c *Client) HintMostRecentCommonAncestor(ctx context.Context, in *pdu.HintMostRecentCommonAncestorReq) (*pdu.HintMostRecentCommonAncestorRes, error) {
-	ctx, endSpan := trace.WithSpan(ctx, "rpc.client.HintMostRecentCommonAncestor")
-	defer endSpan()
-
-	return c.controlClient.HintMostRecentCommonAncestor(ctx, in)
-}
-
 func (c *Client) WaitForConnectivity(ctx context.Context) error {
 	ctx, endSpan := trace.WithSpan(ctx, "rpc.client.WaitForConnectivity")
 	defer endSpan()

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