This change makes getBalance, getCode, getStorageAt, getProof,
call, getTransactionCount return an error if the block number in
the request doesn't exist. getHeaderByNumber still returns null
for missing headers.

ethereum/go-ethereum#16734 introduced BlockHash to the FilterQuery
struct. However, ethclient was not updated to include BlockHash in the actual
RPC request.

ethclient now returns ethereum.NotFound if the server returns null and
no error while accessing blockchain data.

The light client cannot provide arbitrary transactions. The change to
split transaction access into its own interface emphasizes that
transactions should not be relied on and recommends use of logs.

ethclient implements the proposed Ethereum Go API. There are no tests at
the moment, a suite that excercises all implementations of the API will
be added later.