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.
completely optional: it crosses my mind that an export_test helper like

    func (s *State) SetTestHooks(c *gc.C, hooks ...txn.TestHook) whatever-it-returns

might make these a tiny bit more pleasantly compact/readable. up to you.