Add statistics for block filling

[michalkucharczyk]

It would be nice to have a block-filling statistics printed once the executor finishes its work.

Some implementation hints:

• there should be an silent instance of block monitor that collects finalized blocks and keeps track of block number and number of transactions in the block,
• at the end some statistics (also histogram if graphs are enabled) should be printed. Would be good to know how many blocks were filled up to 10%, 25%, 50%, 75%, 90%, 100%. This should probably be doable with percentiles.

This could also be stored in the json journal file, so we can later process data (and draw full graph if needed).