[#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

endpoint/endpoint_zfs_abstraction_step.go

@@ -93,7 +93,7 @@ func HoldStep(ctx context.Context, fs string, v zfs.FilesystemVersion, jobID Job
 		return nil, errors.Wrap(err, "create step bookmark: determine bookmark name")
 	}
 	// idempotently create bookmark
-	err = zfs.ZFSBookmark(ctx, fs, v.ToSendArgVersion(), bmname)
+	stepBookmark, err := zfs.ZFSBookmark(ctx, fs, v, bmname)
 	if err != nil {
 		if err == zfs.ErrBookmarkCloningNotSupported {
 			// TODO we could actually try to find a local snapshot that has the requested GUID
@@ -108,7 +108,7 @@ func HoldStep(ctx context.Context, fs string, v zfs.FilesystemVersion, jobID Job
 	return &bookmarkBasedAbstraction{
 		Type:              AbstractionStepBookmark,
 		FS:                fs,
-		FilesystemVersion: v,
+		FilesystemVersion: stepBookmark,
 		JobID:             jobID,
 	}, nil
 }
@@ -236,7 +236,6 @@ func TryReleaseStepStaleFS(ctx context.Context, fs string, jobID JobID) {
 				Info("destroyed stale step-hold or bookmark")
 		}
 	}
-
 }
 
 var _ BookmarkExtractor = StepBookmarkExtractor