Push RwLock down into OperationPool

There used to be one massive lock on `BeaconChain.op_pool`, however that
would cause unnecessary blocking.

eth2/operation_pool/Cargo.toml

@@ -7,6 +7,7 @@ edition = "2018"
 [dependencies]
 int_to_bytes = { path = "../utils/int_to_bytes" }
 itertools = "0.8"
+parking_lot = "0.7"
 types = { path = "../types" }
 state_processing = { path = "../state_processing" }
 ssz = { path = "../utils/ssz" }