WL#3253: Triggers: multiple triggers per table

Affects: Server-5.7   —   Status: Complete

The server currently supports only one trigger for every action (INSERT,
UPDATE, DELETE) and timing (BEFORE or AFTER). In other words, there might
be at most one trigger for every pair (action, timing).

This WL task is about providing the possibility to have more than one
trigger for every action and timing.