Generalize circuit shorting, now with visibility

[jcorbin]

Add a new "circuits.shorts" config option which should be a Map<String,bool> where each string is a "caller~service~endpoint" tuple with simple wildcarding supported.

This is then used to drive a new ShortedState circuit state which is an UnhealthyState extension which always passes traffic.

cc @kriskowal @rf @raynos

[Raynos]

Seems great. Especially the visibility bit.

[jcorbin]

Now with tests, ready for review.

[jcorbin]

Updated to minimize the new overhead to circuit creation time pointed out by @rf; assumptions being that the abundantly common case will fail the first callerName lookup.

[kriskowal]

:sheep: it