Allow for thread safe DaemonAsync out of the box

[Kayanski]

Today, DaemonAsync is thread safe but submitting transactions with a daemon between threads is not safe because of the account sequence not being updated accordingly when a transaction has been submitted in a block.

This represents the first step : #308

Solution

Wrap the account sequence in a mutex and update it to account for the account sequence change of possible transactions in the same block (is that even possible to have multiple txs in the same block ?)