When a transaction in parallel replication needs to retry (eg. because of
deadlock kill), first wait for all prior transactions to commit before doing
the retry. This way, we avoid the retry once again conflicting with a prior
transaction, requiring yet another retry.

Without this patch, we saw "in the wild" that transactions had to be retried
more than 10 times to succeed, which exceeds the default
--slave_transaction_retries value and is in any case undesirable.

(We already do this in 10.1 in "optimistic" parallel replication mode; this
patch just makes the code use the same logic for "conservative" mode (only
mode in 10.0)).