fix: wait on system.mutations between hosts during migration execution

[kylemumma]

changes

Now when migrations are executed, the migration will not move on from 1 host to the next one until select count(*) from system.mutations where is_done=0 is 0

testing

Not sure how to do a comprehensive test of this but heres how i tested:

• in the case where no mutation in progress, this works. I successfully ran a normal migration locally.
• in the case of a mutation in progress I wasnt able to directly test, but I tested with is_done=1 to make sure the count(*) results were good ([(13,)]) so i think it should work

[nikhars]

LGTM. Agree with Oliver's comment about the exception raised.

[chipzzz]

@kylemumma , thank you for this, on replicated/sharded CH encountering the zookeeper error was painful