Implement Lean BEEFY (#10882)

Simplified BEEFY worker logic based on the invariant that GRANDPA
will always finalize 1st block of each new session, meaning BEEFY
worker is guaranteed to receive finality notification for the
BEEFY mandatory blocks.

Under these conditions the current design is as follows:
- session changes are detected based on BEEFY Digest present in
  BEEFY mandatory blocks,
- on each new session new `Rounds` of voting is created, with old
  rounds being dropped (for gossip rounds, last 3 are still alive
  so votes are still being gossiped),
- after processing finality for a block, the worker votes if
  a new voting target has become available as a result of said
  block finality processing,
- incoming votes as well as self-created votes are processed
  and signed commitments are created for completed BEEFY voting
  rounds,
- the worker votes if a new voting target becomes available
  once a round successfully completes.

On worker startup, the current validator set is retrieved from
the BEEFY pallet. If it is the genesis validator set, worker
starts voting right away considering Block #1 as session start.

Otherwise (not genesis), the worker will vote starting with
mandatory block of the next session.

Later on when we add the BEEFY initial-sync (catch-up) logic,
the worker will sync all past mandatory blocks Signed Commitments
and will be able to start voting right away.

BEEFY mandatory block is the block with header containing the BEEFY
`AuthoritiesChange` Digest, this block is guaranteed to be finalized
by GRANDPA.

This session-boundary block is signed by the ending-session's
validator set. Next blocks will be signed by the new session's
validator set. This behavior is consistent with what GRANDPA does
as well.

Also drop the limit N on active gossip rounds. In an adversarial
network, a bad actor could create and gossip N invalid votes with
round numbers larger than the current correct round number. This
would lead to votes for correct rounds to no longer be gossiped.

Add unit-tests for all components, including full voter consensus
tests.

Signed-off-by: Adrian Catangiu <[email protected]>
Co-authored-by: Tomasz Drwięga <[email protected]>
Co-authored-by: David Salami <Wizdave97>