Update getaddressbalance RPC method for block explorer usage

[mpguerra]

Motivation

In order to ensure that zebra is ready to act as a zcashd replacement, we need to implement the RPC methods required to support block explorers.

We currently support lightwalletd usage of the getaddressbalance RPC, however:

1. zcashd accepts a single string parameter as well as an array of strings as an argument. Zebra only accepts an array of strings because lightwalletd always calls this RPC with an array of addresses.
2. zcashd returns the total amount of Zatoshis received by the addresses, but Zebra doesn't because lightwalletd doesn't use that information.

We need to:

1. Check whether we need to update the getaddressbalance RPC method to fully match zcashd for usecases other than lightwalletd
2. If so, implement the required changes to support the relevant usecases.

Specification

https://zcash.github.io/rpc/getaddressbalance.html

[arya2]

We need a db format upgrade for this one to add the received field.