Change Panel api to not expose Circuits, have Get return ok; HTTPClient uses a panel
Rather than exposing the Circuits map in Panel in an unsafe way, all access is done
via the Get function. If the breaker is not found it will still return the NopBreaker,
but it will also return false for the 'ok' value, similar to the way map lookups work.
The circuits map then is wrapped in a RWMutex for safer access.

Update the http clients to use a circuit panel rather than a default breaker and their
own map of breakers.

Closes #17