In order to follow REST a bit more carefully, all write operations
should go through a POST in the future. We still allow read operations
through either GET or POST (similar to the Graphite /render endpoint),
but write operations will trigger a returned warning as part of the JSON
response and will eventually return an error.
Also updates the Golang client libraries to always use POST instead of
GET.
Fixes#6290.
6f61c0ea4a