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.
