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
