Once a client is reportedly gone, processing its VCL task(s) is just a
waste of resources. The execution of client-facing VCL is intercepted
and an artificial return(fail) is returned in that scenario.

Thanks to the introduction of the universal return(fail) proper error
handling and resource tear down is already in place, which makes this
change safe modulus unknown bugs. This adds a circuit breaker anywhere
in the client state machine where there is VCL execution.

A new Reset time stamp is logged to convey when a task does not complete
because the client is gone. This is a good complement to the walk away
feature and its original circuit breaker for the waiting list, but this
has not been integrated yet.

While the request is technically failed, it won't increase the vcl_fail
counter, and a new req_reset counter is incremented. This new behavior
is guarded by a new vcl_req_reset feature flag, enabled by default.

Refs #3835
Refs 61a15cbf
Refs e5efc2c8
Refs ba54dc91
Refs 6f50a00f
Refs b8816994