Clean up transactions in maintenance scripts

Clean up transactions in maintenance scripts

Add transaction methods to complement getDB().
This makes it easy to grep for direct begin()/commit()
calls to IDatabase by having script use their own
wrapper. Maintenance scripts are one of the few places
that can (and need to) use begin/commit instead of the
start/end atomic methods.

Eventually, there should be almost no direct callers
and those methods can be made stricter about throwing
errors on nested calls.

Change-Id: Ibbfc7a77c0d2a55f7fc2261087f6c3a19061e0aa