SimulatedBackend doesn't match ethereum interface and ethclient

[s1na]

As far as I understand the goal is that the interfaces defined here match that of ethclient and SimulatedBackend so it's easy to swap out a real node RPC server with a simulated blockchain. However there are currently a few differences preventing this:

• PendingBalanceAt, PendingStorageAt and PendingTransactionCount are not defined on the simulated backend
• SubscribePendingTransactions is part of the interfaces, but it is not defined on the simulated backend. Also it is defined on gethclient, not directly on ethclient.

[bnovil]

I have the same thought. It seems better that SimulatedBackend implements the interface PendingStateReader.