possible new methods
Merge commit '56b23310dc892050c8de94c0df8e701c1efa6c42' into streamline-api
health refactor. update health via Go method.

make health updates a simple bool chan.
infer time when channel is read.
remove mutex from health since all updates are currently happening in Monitor
add cleanup of old health buckets.
make circuit open/closed an explicit state to avoid recalculating health for each execution
remove dead code
test for circuit toggling into closed state