[#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

zfs/versions.go

@@ -116,6 +116,13 @@ func (v FilesystemVersion) RelName() string {
 }
 func (v FilesystemVersion) String() string { return v.RelName() }
 
+// Only takes into account those attributes of FilesystemVersion that
+// are immutable over time in ZFS.
+func FilesystemVersionEqualIdentity(a, b FilesystemVersion) bool {
+	// .Name is mutable
+	return a.Guid == b.Guid && a.CreateTXG == b.CreateTXG && a.Creation == b.Creation
+}
+
 func (v FilesystemVersion) ToAbsPath(p *DatasetPath) string {
 	var b bytes.Buffer
 	b.WriteString(p.ToString())