Add BEEFY latestFinalized RPC and deduplicate code between BEEFY and GRANDPA (#10568)

* beefy: add dummy latest_finalized() RPC

* beefy: rpc latest_best_beefy() using shared mem

* beefy: rpc populate latest_best_beefy()

* beefy: rpc handle readiness

* beefy: best block over channel - wip

Not working because channel can't be simply opened and receiver passed
to `rpc_extensions_builder` because `rpc_extensions_builder` has to be
`Fn` and not `FnOnce`... and and Receiver side of mpsc can't be cloned

yay!..

* beefy: make notification channels payload-agnostic

* beefy: use notification mechanism instead of custom channel

* beefy: add tracing key to notif channels

* sc-utils: add notification channel - wip

* beefy: use sc-utils generic notification channel

* grandpa: use sc-utils generic notification channel

* fix grumbles

* beefy-rpc: get best block header instead of number

* beefy-rpc: rename to `beefy_getFinalizedHead`

* fix nitpicks

* client-rpc-notifications: move generic Error from struct to fn

* beefy: use header from notification instead of getting from database

* beefy-rpc: get best block hash instead of header

* beefy-rpc: fix and improve latestHead test

* beefy-rpc: bubble up errors from rpc-handler instantiation

* update lockfile

* Apply suggestions from code review

Co-authored-by: André Silva <123550+andresilva@users.noreply.github.com>

* fix errors and warnings

* fix nit

Co-authored-by: André Silva <123550+andresilva@users.noreply.github.com>

substrate/client/utils/Cargo.toml

@@ -12,9 +12,13 @@ readme = "README.md"
 [dependencies]
 futures = "0.3.9"
 lazy_static = "1.4.0"
+parking_lot = "0.11"
 prometheus = { version = "0.13.0", default-features = false }
 futures-timer = "3.0.2"
 
 [features]
 default = ["metered"]
 metered = []
+
+[dev-dependencies]
+tokio-test = "0.4.2"