* Add `CircuitBreaker` abstraction (with half-open state) and add circuit breaker state change event observer support.
* Improve circuit breaking strategy (avg RT → slow request ratio) and make statistics of each rule dependent (to support arbitrary statistic interval).
* Add simple "trial" mechanism (aka. half-open).
* Refactor mechanism of metric recording and state change handling for circuit breakers: record RT and error when requests have completed (i.e. `onExit`, based on #1420).

Signed-off-by: Eric Zhao <sczyh16@gmail.com>