Allow dynamic failure execution threshold in circuit breakers

[jhalterman]

When using time based thresholding in circuit breakers, the failure execution threshold influences how much trial load is allowed in when in a half-open state. Getting this number right likely depends on the normal volume of executions that go through the circuit breaker, so it makes sense that this setting could be dynamic/computed, where a user could provide a value that is based on the recent execution volume. This way we don't risk over or under-loading the breaker in the half-open state.