Bug 18894: Add ability to limit the number of messages sent by process_message_queue...

Bug 18894: Add ability to limit the number of messages sent by process_message_queue.pl at a time

Having the ability to limit the number of messages sent by process_message_queue.pl on a single run would be very useful for controlling home many messages are sent at a given time. This can help prevent too many messages being sent out at once and getting flagged as a spammer.

Test Plan:
1) Apply this patch
2) Generate some number of messages in the message queue
3) Run process_message_queue.pl with the new --limit option,
   set limit to a number smaller than the number of pending messages
4) After the script has run, check the database and note that only
   a number of pending messages were sent, and that the remaining amount
   of pending messages is the original amount less the number specified
   as the limit

Signed-off-by: Mark Tompsett <mtompset@hotmail.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>