Ability to manually clear in-memory cache

[aaricpittman]

Is your feature request related to a problem? Please describe.

When in ldd mode, I would like to be able to manually clear the in-memory cache rather than wait for the expiration.

Describe the solution you'd like

The ability to call client.clear_cache or store.clear_cache or something to that effect which would ultimately call LaunchDarkly::ExpiringCache#clear.

Describe alternatives you've considered

Given the encapsulation of the cache, the only option currently seems to be to close the current client and initialize a new one.