Bug 31453: Add ability to filter messages to process using process_message_queue...

Bug 31453: Add ability to filter messages to process using process_message_queue.pl via a command line parameter

It would be very useful to be able to tell process_message_queue.pl to skip processing some messages. This is particularly useful where a plugin handles sending some message using the before_send_messages hook, but while that plugin is processing, more messages meant for the plugin might be queued. At that point, control moves back to the script and SendQueuedMessages is called, and those messages end up being processed there instead of by the plugin.

Test Plan:
1) Apply this patch
2) Queue two messages, each with a unique word
3) Run process_message_queue --where "content NOT LIKE '%WORD%'"
   where WORD is a unique word in one of the two message
4) Note the message containing "WORD" was not processed
5) prove t/db_dependent/Letters.t

Signed-off-by: Andrew Fuerste-Henry <andrewfh@dubcolib.org>
Signed-off-by: Victor Grousset/tuxayo <victor@tuxayo.net>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>