Get available treasury balance endpoint.

[Sirmorrison]

We need a method for getting the available treasury balance. This seems to have been implemented on dcrd.
https://github.com/decred/decred-binaries/releases#dcrd-v160-rc1

[crux25]

#255 brings in the capability to fetch Treasury balance from the backend service.

[monsa00]

related to planetdecred/godcr#657