kvstore API: implement /api/get with HTTP GET

[rg0now]

Currently /api/get accepts HTTP POST requests only and expects the key in the request body, which is not really intuitive. This should be re-implemented over HTTP GET, accepting the key in a query parameter or as the last component of the HTTP path (i.e., like /api/get/key).

This issue tracks the progress on this change.

[benjoe1126]

Will start working on it