[#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.go

@@ -54,6 +54,30 @@ type Abstraction interface {
 	json.Marshaler
 }
 
+func AbstractionEquals(a, b Abstraction) bool {
+	if (a != nil) != (b != nil) {
+		return false
+	}
+	if a == nil && b == nil {
+		return true
+	}
+	var aJobId, bJobId JobID
+	if aJid := a.GetJobID(); aJid != nil {
+		aJobId = *aJid
+	}
+	if bJid := b.GetJobID(); bJid != nil {
+		bJobId = *bJid
+	}
+	return a.GetType() == b.GetType() &&
+		a.GetFS() == b.GetFS() &&
+		a.GetName() == b.GetName() &&
+		a.GetFullPath() == b.GetFullPath() &&
+		aJobId == bJobId &&
+		a.GetCreateTXG() == b.GetCreateTXG() &&
+		zfs.FilesystemVersionEqualIdentity(a.GetFilesystemVersion(), b.GetFilesystemVersion()) &&
+		a.String() == b.String()
+}
+
 func (t AbstractionType) Validate() error {
 	switch t {
 	case AbstractionStepBookmark: