RPC:SendSignedTransaction can be optimized

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.