* Add stale branches heads to finality notifications

Warning. Previous implementation was sending a notification for
each block between the previous (explicitly) finalized block and
the new finalized one (with an hardcoded limit of 256).

Now finality notification is sent only for the new finalized head and it
contains the hash of the new finalized head, new finalized head header,
a list of all the implicitly finalized blocks and a list of stale
branches heads (i.e. the branches heads that are not part of the
canonical chain anymore).

* Add implicitly finalized blocks list to `ChainEvent::Finalized` message

The list contains all the blocks between the previously finalized block
up to the parent of the currently finalized one, sorted by block number.

`Finalized` messages handler, part of the `MaintainedTransactionPool`
implementation for `BasicPool`, still propagate full set of finalized
blocks to the txpool by iterating over implicitly finalized blocks list.

* Rust fmt

* Greedy evaluation of `stale_heads` during finalization

* Fix outdated assumption in a comment

* Removed a test optimization that is no more relevant

The loop was there to prevent sending to
`peer.network.on_block_finalized` the full list of finalized blocks.

Now only the finalized heads are received.

* Last finalized block lookup not required anymore

* Tests for block finality notifications payloads

* Document a bit tricky condition to avoid duplicate finalization notifications

* More idiomatic way to skip an iterator entry

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

* Cargo fmt iteration

* Typo fix

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

* Fix potential failure when a finalized orphan block is imported

* Apply suggestions from code review

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

* Apply suggestions from code review

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>