Incremental backoff for `local_block_broadcaster`

[pwojcikdev]

This implements incremental backoff for block rebroadcasting. Previously block rebroadcasts happened every 60 seconds. This changes it, so that early on rebroadcasts happen more frequently, with 3 second interval increase. Ie. first rebroadcast happens after 3 seconds, second after 9, third after 18 etc.

Additionally there are improvements that were necessary to implement the above: avoiding unnecessary locking and config cleanup.