In order to have more control over the mempool implementation,
introduce a new exported function RemoveTxByKey.
Export also TxKey() and TxKeySize. Use TxKeySize const instead of
sha256.size, so future changes on the hash function won't break the API.
Allows using a TxKey (32 bytes reference) as parameter instead of
the complete array set. So the application layer does not need to
keep track of the whole transaction but only of the sha256 hash (32 bytes).
This function is useful when mempool.Recheck is disabled.
Allows the Application layer to implement its own cleaning mechanism
without having to re-implement the whole mempool interface.
Mempool.Update() would probably also need to change from txBytes to txKey,
but that would require to change the Interface thus will break backwards
compatibility. For now RemoveTxByKey() looks like a good compromise,
it won't break anything and will help to solve some mempool issues from the
application layer.
Signed-off-by: p4u <pau@dabax.net>