Make state-modifying API calls POSTs
Created by: VoidingWarranties
This PR is intended to serve as an area for discussion about API improvements, not just this specific suggested improvement. As we approach v1.0.0 we need to finalize the API. The change in this PR (so far) is to make every state-modifying call a POST and all other calls a GET.
Another proposed API change we have talked about offline before is adding more HTTP verbs. DELETE in particular would be more semantic for the renter / hostdb / gateway when deleting or removing files / hosts / peers.