Here we add a new BenchmarkBoltBatching test that helps us benchmark the
performance when writing to a bbolt backend using various different
configurations. We test using N txs for N writes, 1 tx for N writes and
then various configurations when using the TimeScheduler.
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.
