fix/chain_head: Ensure correct events for finalized branch (#13632)

* chain_head/follow: Ensure correct events for finalized branch

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* Reenable tests

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* Do some clean ups and add some more docs

* Fix gramatic

* Update client/rpc-spec-v2/src/chain_head/chain_head_follow.rs

Co-authored-by: Sebastian Kunert <skunert49@gmail.com>

* rpc/chain_head: Introduce error for absent headers

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

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Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>
Co-authored-by: Bastian Köcher <info@kchr.de>
Co-authored-by: Bastian Köcher <git@kchr.de>
Co-authored-by: Sebastian Kunert <skunert49@gmail.com>