Converting schema commands to RPCs.

Originally I wanted to use streaming RPCs, but there is nothing
to communicate back while the changes are applied, so no reason.
Converting Snapshot to streaming RPC call.
Now converting ReserveForRestore and SnapshotSourceEnd to RPC.
Converting Restore, MultiSnapshot and MultiRestore to streaming RPCs.
Remove ping, rename rpcping to ping.

(action nodes are not used any more, but not cleaned up yet).
Merge branch 'master' into replication
Moving a function out.
Changing syntax of streaming RPC returns.

So other RPC systems can convert stuctures if they need to.