Make sure that pending transactions are not lost on restart

[Geod24]

With MainNet around the corner, we will sometimes need to update sebak all nodes at once.
We should make sure that nodes do not loose their pending txs on shutdown.

[anarcher]

I think that we need a tx pool with WAL ( or to keep in file or persistend keep when add)

[spikeekips]

In the current scenario, the lost transactions could not be found thru /api/v1/transactions/{hash}, so the client fails to check, they will send it again. This feature will make sebak to be more stable.

[anarcher]

Without persisted txs in the pool, Delayed txs is OK?

[Geod24]

Delayed ops will have to be saved as well, but they were reverted so it's not a problem ATM.