Check functionality for RPC methods depending on the `pending` tag

[rodrigo-o]

Right now the block number resolver which may receive a tag works returning always the latest block, BUT every RPC call will probably need to have different logic for pending. For example, eth_getTransactionCount instead of getting the nonce for the tie it gets it from the mempool first and just defaulting to the latest block later (which was needed for #1092). As far as i know that's the only RPC method which is handling pendings as expected for now, we need to check how about the others.

This might become an umbrella task if every one of those methods proves to be complex in itself.