Support neutrino backend for mobile devices. #68
Description
Mobile devices have limited space and bandwidth, so that syncing full Bitcoin/Omnilayer database is not possible.
obd currently uses omnicore-proxy as a full node backend, but omnicore does not support neutrino mode yet. So that if obd runs on a mobile phone, it will download full blocks data to the device, which is too much.
The solution is: maintain two full nodes as backends, one is netrino bitcoin node, which is used to sync data to mobile obd wallet. And the other is omnicore full node, which is used to manipulate on-chain omni assets.