mysql-predump optimizations

Use bash instead of dash for its job control features. Do not use wait
since that would arbitrarily wait on a single job which might take long
to finish leading to a worst case scenario of 1 job in case the rest
finished before it. Unconditionally wait for all of the jobs to finish
before returning. set -x to make sure an error in the script fails the
entire backup. This allows for more consistent backups

Change-Id: I771058f4415186b047b0238265646d6b52edec6e