Add a re-usable implementation of the sqldb.TxOptions interface and make
use of this in the various spots (invoices, batch and graph/db) where we
had previously defined individual implementations that were all doing
the same thing.
In this commit, we update the batch schedular so that it has the ability
to do read-only calls. It will do a best effort attempt at keeping a
transaction in read-only mode and then if any requests get added to a
batch that require a read-write tx, then the entire batch's tx will be
upgraded to use a read-write tx.
In preparation for using the same logic for non-bbolt backends, we adapt
the batch.Schedular to be more generic.
The only user of the scheduler at the moment is the KVStore in the
`graph.db` package. This store instantiates the bbolt implementation of
the scheduler.
