moves caching to wrapper class 'FetchAuthTokenCache' and adds Memory cache

[bshaffer]

Addresses #121

Highlights:

• Adds FetchAuthTokenCache wrapper class to implement caching on anything implementing FetchAuthTokenInterface.
• Removes caching from all Middleware / Subscriber classes
• Bypasses caching (by setting the key to null) for AppIdentityCredentials - Caching here is handled by the AppIdentityService
• Removes calls to getCacheKey from Cache Trait - that function does not exist as part of the trait, and as a result was causing some unexpected problems. The cache key is now explicitly passed in.

[bshaffer]

@dwsupplee and @jdpedrie I would love it if you could take a look as well!

[tmatsuo]

LGTM, I have one question but it can be left as TODO and you can file another PR.

[bshaffer]

As discussed, we should add an in-memory implementation for PSR-6 (for example) to this library to use as a default caching mechanism.

This can be consumed by googleapis/gax-php and googlecloudplatform/gcloud-php

[dwsupplee]

I've opened #126 against this PR which includes an in-memory implementation. Let me know what you think :).

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[dwsupplee]

I consent

[bshaffer]

yeah it doesn't seem to work quite right in this scenario 😄