Add L2 block sequential process. Execute empty L2 block. Add tracking number to L2 block. Several refactors

[agnusmor]

What does this PR do?

• Added L2 block sequential process mode. This mode can be enabled using the following config parameter:

[Sequencer]
[Sequencer.Finalizer]
SequentialProcessL2Block = true

• Execute always the changeL2Block tx when opening a new WIP L2 block (instead to wait to execute with the first tx of the block)
• Added a tracking number to L2 blocks to ease following the L2 block process/store in the logs
• Several minor refactors

Reviewers

Main reviewers:

@ToniRamirezM