Allow save inventory to skip starting a transaction if the record being
saved was not actually changed by the provider's inventory data.

VMs and Storages still directly call `update_attributes!`.  At the
moment, it doesn't seem like those object counts are limiting factors.
If that changes, we can look at applying the same change there.

Do not do a blank transaction if the record hasn't changed,
becaiuse it causes extra 2 queries, transaction BEGIN and END

Wrap updating&deleting in 1 big transaction. Doing transaction per
update is expensive, since we do BEGIN, update, END, so 3 queries
per one updateand the same for delete. With remote DB, this will
add a lot of processing time.

Add a comment about rails issue causing nil transaction

Do a transaction only if there are items to be deleted, to
avoind empty transactions.