Support custom app health function

[hsiliev]

#601 proposes /health endpoint. Usually we wrap endpoints with breaker to use performance counters and feed it with data to open in case of a problem.

To have long-running apps health in the perf. counters we need to support time-window bigger than several seconds. This leads to increased memory consumption and OOM.

Therefore we might simply keep a status (ok, failed?) to reduce the memory consumption to minimum. Another idea would be to allow breaker to be controlled with custom function.

[cf-gitbot]

We have created an issue in Pivotal Tracker to manage this:

https://www.pivotaltracker.com/story/show/143532971

The labels on this github issue will be updated when the story is started.