Basic PoT gossip

[nazar-pc]

Now that verification in #1918 is done in its initial form things start moving a bit faster.

First commit adds support for node restart, we just initialize timekeeper from the last known block on restart. There is no point in doing anything fancier since node is likely way behind anyway.

Second commit refactors old gossip code to work with input/output streams instead, extends PotSource to follow block import (such that even if timekeeper is not enabled and we don't get gossip, we still update local state). This commit also made Timekeeper respect the boolean flag (previously was running unconditionally).

There are still many issues and potential improvements around gossip, Timekeeper reset is not yet supported, but most of these things have TODOs in the code now, so can be followed from there.

Last commit removed old timekeeper and state management code as it became clear they will not be necessary.

Next steps:

• add support for timekeeper reset (and generally be prepare it to support entropy injection and slot iterations adjustment)
• add entropy injection

I may also do some cleanups, refactoring and smaller things in the meantime as well, will see if I get stuck somewhere.

Code contributor checklist:

• I have read, understood and followed contributing guide

[nazar-pc]

No one reviewed this yet, so I rebased it on main