Put all tags for a given instance into a list for a single batch insert

Addresses #414 by using the execBatch feature of QSQLQuery.

Might still be improved if we can do other operations as batch inserts
as well.
Merge pull request #415 from pieper/414-batch-insert-tags

Put all tags for a given instance into a list for a single batch insert

Thanks for the reviews @nolden and @jcfr - I'm going to go ahead and do the merge (same tests fail as before ;( so no harm from this pull.