vacuum: introduce the RECEIVE_RECORDS control

vacuum: introduce the RECEIVE_RECORDS control

This in preparation of turning the vacuming on the lmaster into
into a two phase process:

- First the node sends the list of records to be vacuumed
  to all other nodes with this new RECEIVE_RECORDS control.
  The remote nodes should store the lmaster's empty current copy.
- Only those records that could be stored on all other nodes
  are processed further. They are send to all other nodes with
  the TRY_DELETE_RECORDS control as before for deletion.

Signed-off-by: Michael Adam <obnox@samba.org>
Reviewed-By: Amitay Isaacs <amitay@gmail.com>