ctdb: Adding memory pool for queue callback

ctdb: Adding memory pool for queue callback

The received packet is copied into a newly allocated memory chunk for further
processing by the assigned callback. Once this is done, the memory is free'd.
This is repeated for each received packet making the memory allocation / free
an expensive task. To optimize this process, a memory pool is defined which
is sized identically to the queue's buffer.
During tests it could be seen that more than 95% of all messages were sized
below the standard buffer_size of 1k.

Signed-off-by: Swen Schillig <swen@vnet.ibm.com>
Reviewed-by: Martin Schwenke <martin@meltin.net>
Reviewed-by: Christof Schmitt <cs@samba.org>
Autobuild-User(master): Christof Schmitt <cs@samba.org>
Autobuild-Date(master): Fri Dec  7 23:27:16 CET 2018 on sn-devel-144