ctdb-call: Convert pending calls list to per database list

ctdb-call: Convert pending calls list to per database list

The pending calls are migration requests received from clients (over unix
domain socket) which are under processing.  After a recovery is finished,
any requests which are under processing will be dropped since they do
not belong to the current generation.  All the pending call requests
are resent with new generation to restart record migrations.

This is in preparation for parallel database recovery.

Signed-off-by: Amitay Isaacs <amitay@gmail.com>
Reviewed-by: Martin Schwenke <martin@meltin.net>