sendTransaction method of RPC should validate and reject invalid transaction earlier

[jangko]

Instead of waiting the transaction to be validated by the executor, the RPC should reject and respond with error.
This need refactor validateTransaction into complete and basic validation.

basic ValidateTransaction is a helper method to check whether a transaction is valid
according to the consensus rules, but does not check state-dependent validation
(balance, nonce, etc).

The txpool should use this basic transaction validation and reject immediately invalid txs.
The RPC method should query the rejection status from txpool and see if it's rejected.

move inPoolAndOk from graphql/ethapi.nim and sync/handlers/eth.nim to txpool.