Include consensus info in most GET requests

[lukechampine]

For example, GET /balance would also return the current height and ID. This would allow the balance (and transactions, file contracts, etc.) to be checked "atomically" -- there is no uncertainty about which block height the reported information corresponds to. Currently, if you want that certainty, you must call /consensus, then /balance, then /consensus again, and check that the IDs match. If they don't, you must repeat the sequence until they do.

I'm wondering if there is a less intrusive way of including this information along with the response. Could it be placed in a header? It's not a big deal, but it irks me that calling /balance would return an object with a field called "balance" rather than simply returning the balance.

[lukechampine]

Note that, on the server side, we'll still need to do the consensus-balance-consensus loop when calling the wallet methods. (Otherwise, all the wallet methods would need to return consensus info as well.)