[refactor] improve synchronizer and bulk block processing

[akichidis]

This PR is:

• making the synchronzer continuously sending blocks to peers and not attempt send only when a new block is out.
• sample peers based on latency to ensure that lower latency peers are prioritised (there might be some exceptions to that)
• make the send operations on the block disseminator non blocking - so we avoid clogging the network loops
• send multiple blocks at once and process them in bulk and pre-check if they have been already processed so we avoid any penalties from verification etc.