RPC:SendSignedTransaction can be optimized

[HoOngEe]

When the RPC SendSignedTransaction is called, the client tries to construct SignedTransaction from an untrustedRlp. It carries out a computationally heavy job (e.g. public key recovery). However, add_transactions_to_pool accepts only UnVerifiedTransaction, SignedTransaction is converted to the untrusted one again. so the signature verification step should be repeated again.
We can reduce the repeated verification steps.