[refactor] remove json_spirit dep (replace with UniValue)

[rikublock]

json_spirit was replaced with UniValue in bitcoin core v0.12.0, because it is more lightweight and reduces compile time. Currently, Blocknet/xbridge uses a mix of both. This causes compatibility issues and makes it difficult to change/update certain sections in the code.

Any remaining usage of json_spirit should replaced with UniValue to resolve those issues.