An abstraction for transaction pool.
This trait is used by offchain calls to be able to submit transactions.
The main use case is for offchain workers, to feed back the results of computations,
but since the transaction pool access is a separate
ExternalitiesExtension it can
be also used in context of other offchain calls. For one may generate and submit
a transaction for some misbehavior reports (say equivocation).
The transaction will end up in the pool and be propagated to others.