Change block execution to be able to run for a period of time

[AurelienFT]

Current behavior

We are waiting in PoA for the Instant for next block creation then we are asking block executor to create it.

Evolution

We want PoA to directly inform executor that the time window before the next block has started (with a timeout on the block time) and that executor can start execute transactions that he can get from the pool until the timeout is reached. For each transaction executed we want to inform the consensus module for signing a new pre-confirmation message.

Changes to the code

• Changes on interval computation on PoA
• Two new arguments pass to the block executor : a waiter for new transactions during the block time, a channel to send pre-confirmation after execution.