The current interface supports only one method for both crafting and
publishing a batch transaction, SubmitBatchTx. Currently this is being
executed on each new gas price that the txmgr commands, implying we are
doing a lot of extra work to rederive batches. In addition, much our
instrumentation lives inside this method, meaning that the they are also
being recorded multiple times per transaction. When we get to
processing larger batches on Kovan and Mainnet, this could also become a
resource bottleneck.

This commit remedies all of the above issues by splitting out the
transaction crafting process from the publication. A new method,
CraftBatchTx, is added to the Driver interface. This method is
responsible for creating a fully formed transaction, but does not
publish it. The responsibility of SubmitBatchTx is now modified to take
an predefined transaction, i.e. created by CraftBatchTx, overwrite the
supplied gas price, and publish. In this way, this expensive call to
build batches can be done once before handing the transaction of the
txmgr.