Improve GGRC.RequestStore

* Encapsulate better
* Allow simultaneous record and replay (to handle request signature
  changing due to timing issues with replay.)
* Don't auto-reload when changing state
* Limit cached requests to just "GET" requests
* Change API:  GGRC.RequestStore.<method>:
  - `set_record(true/false)`
  - `set_replay(true/false)`
  - `pause()` -> `set_record(false); set_replay(false)`
  - `clear()`