variable settings (rpl_sys)

Problem: under certain conditions (e.g. user variables usage in triggers)
accessing a user defined variable we may use a variables hash table that
belongs to already deleted thread. It happens if
thd= new THD;
has the same address as just deleted thd as we use
if (stored_thd == thd)
to check.
That may lead to unpredictable results, server crash etc.

Fix: use thread_id instead of thd address to distinguish threads.

Note: no simple and repeatable test case.