New txn package

Introduce a new state/txn package which contains the transaction functionality extracted from state/State. A new TransactionRunner interface is implemented which provides methods to i) run transactions directly without retries, ii) invoke a callback to get transactions to run and retry if there is contention. All the hooks and other related functionality is moved out of State. The new txn runner can now be used standalone outside of the State instance, and is also embedded in State.

The various callsites are all migrated to use the new infrastructure. A few unit tests are also added for the hooks functionality - these were missing before.
ok, I can accept it as a behaviour-preserving refactor, but my impression had been that a whole bunch of these methods were using ErrorContextf to annotate ErrExcessiveContention (and that that particular error wasn't actually leaking out -- but I guess that's wrong, given that it has a code in params).