Watch existing extrinsics RPC (#3873)

* Transaction pool watch intristics.

* Track extrinsic rpc methods.

* Test for pool watcher.

* Track extrinsic rpc test.

* Fix rpc naming.

* review fixes

* Update jsonrpc and use une subcription.

* Naming and dependencies.

substrate/node/rpc/Cargo.toml

@@ -6,7 +6,7 @@ edition = "2018"
 
 [dependencies]
 client = { package = "substrate-client", path = "../../core/client" }
-jsonrpc-core = "13.2.0"
+jsonrpc-core = "14.0"
 node-primitives = { path = "../primitives" }
 sr-primitives = { path = "../../core/sr-primitives" }
 srml-contracts-rpc = { path = "../../srml/contracts/rpc/" }