lncli: use original snake_case names for JSON serializing
lncli: use original snake_case names for JSON serializing

Because we now use printRespJSON everywhere where we print RPC
responses as JSON, we can simply instruct the jsonpb marshaler to
use the original snake_case name specified in the proto file for
the JSON field names and not the default camelCase.

aakselrod/lnd,LightningNetwork/lnd,LightningNetwork/lnd,aakselrod/lnd