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/Cargo.lock

@@ -323,6 +323,27 @@ dependencies = [
  "trust-dns-resolver",
 ]
 
+[[package]]
+name = "async-stream"
+version = "0.3.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "171374e7e3b2504e0e5236e3b59260560f9fe94bfe9ac39ba5e4e929c5590625"
+dependencies = [
+ "async-stream-impl",
+ "futures-core",
+]
+
+[[package]]
+name = "async-stream-impl"
+version = "0.3.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "648ed8c8d2ce5409ccd57453d9d1b214b342a0d69376a6feda1fd6cae3299308"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn",
+]
+
 [[package]]
 name = "async-task"
 version = "4.0.3"
@@ -508,6 +529,7 @@ version = "4.0.0-dev"
 dependencies = [
  "beefy-gadget",
  "beefy-primitives",
+ "derive_more",
  "futures 0.3.16",
  "jsonrpc-core",
  "jsonrpc-core-client",
@@ -515,12 +537,15 @@ dependencies = [
  "jsonrpc-pubsub",
  "log 0.4.14",
  "parity-scale-codec",
+ "parking_lot 0.11.2",
  "sc-rpc",
+ "sc-utils",
  "serde",
  "serde_json",
  "sp-core",
  "sp-runtime",
  "substrate-test-runtime-client",
+ "thiserror",
 ]
 
 [[package]]
@@ -8743,7 +8768,9 @@ dependencies = [
  "futures 0.3.16",
  "futures-timer",
  "lazy_static",
+ "parking_lot 0.11.2",
  "prometheus",
+ "tokio-test",
 ]
 
 [[package]]
@@ -10745,6 +10772,19 @@ dependencies = [
  "tokio-reactor",
 ]
 
+[[package]]
+name = "tokio-test"
+version = "0.4.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "53474327ae5e166530d17f2d956afcb4f8a004de581b3cae10f12006bc8163e3"
+dependencies = [
+ "async-stream",
+ "bytes 1.1.0",
+ "futures-core",
+ "tokio",
+ "tokio-stream",
+]
+
 [[package]]
 name = "tokio-tls"
 version = "0.2.1"