This patch includes following fixes:
 - messages may be now sent when database transactions active
 - consistent return status on failure from message_send(), false is
   returned only when message not created in message(_read)? table,
   processor failure results in debugging message only and messages
   are not marked as read
 - message_sent is triggered always with id from message table
 - logic for marking messages as viewed was standardised
 - message_viewed event is triggered consistently
 - improved performance when fetching user preferences
 - full unit tests coverage for send_message() function
 - fixed multiple other smaller issues discovered by unit tests

Conflicts:
	theme/bootstrapbase/style/moodle.css

Call can_edit() instead of repeating a (slightly wrong) access check.

The pass by reference meant the function was clobbering the variable in
the calling function. Once that was change, it was not strictly
necessary to rename the variable in the function, but I did anyway for
clarity.

Thanks to Rajesh Taneja for finding the problem, and identifying which
bit of code was broken. I really just made the patch.