WL#5944: Semi-safe stop of slave threads

==== Background ====

The slave threads can be stopped abruptly (KILL) or safely (STOP SLAVE). When the 
slave is stopped abruptly, and there are ongoing transactions containing non-
transactional updates, the slave may be left in an inconsistent state. When the 
slave is stopped safely, it may take a while to stop the slave. STOP SLAVE may 
even hang, e.g., if the connection with the master is cut.

==== Feature ====

In this worklog, we implement the following feature:
Try to stop the slave safely for X seconds. If the slave is still running after X 
seconds, stop the slave abruptly.