Replace internal Hystrix counters with HdrHistogram

[mattrjacobs]

HdrHistogram implements the same functionality as the internal Hystrix counters, but with better resolution/space/speed guarantees.

I'm not sure which APIs would change as a result of this switch yet, but it should be fairly minimal.

[mattrjacobs]

#779 replaced AtomicIntegerArrays with HdrHistogram (specifically IntCountsHistogram) for latency metrics

[mattrjacobs]

In the course of doing this upgrade, I inadvertantly introduced thread safety issues and a memory leak. I rolled back this change for now, while I consider how to reintroduce it.

[mattrjacobs]

#843 provides a facility to vary metrics collection behavior. That will make this feature easier to implement. My current plan is to introduce this as an option in 1.5.x, and hopefully receive feedback (from internal testing and external users) to determine if it should be the default going forward.

[mattrjacobs]

Closing in favor of more-general #943