The patch is inspired from MySQL. Instead of using a single String to
hold the current active debug_sync signal, use a Hash_set to store
LEX_STRINGS. This patch ensures that a signal can not be lost, by being
overwritten by another thread via set DEBUG_SYNC = '... SIGNAL ...';

All signals are kepts "alive" until they are consumed by a wait event.

This requires updating test cases that assume the GLOBAL signal is never
consumed.

Follow-up work needed:
Port the additional syntax that allows one to set multiple signals
and also conditionally deactivate signals when waiting.