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.
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.
Currently, a few of the graph KVStore methods take the
`batch.SchedulerOptions` param. This is only used to set the LazyAdd
option. A SchedulerOption is a functional option that takes a
`batch.Request` which has bolt-specific fields in it. This commit
restructures things a bit such that the `batch.Request` type is no
longer part of the `batch.SchedulerOptions` - this will make it easier
to implement the graph store with a different DB backend.
