Remove hard-coded pivot and set it basing on CL message

[marcindsobczak]

Is your feature request related to a problem? Please describe.
Before we even start syncing, we need to download blocks between pivot and current head. It's not a problem just after releases when hard-coded pivot is close to chain head, but after few days/weeks it significantly slow us down - downloading post-pivot blocks can make our syncing process with 2-3 weeks old pivot even 30-40% longer.

Describe the solution you'd like
We can remove hard-coded pivot (or left it as it is, but overwrite) and set pivot basing on message from CL - e.g. use finalized block from FCU message.