TServerFramework::stop may fail to interrupt connected clients

This is a sequence that exposes the race condition:
(1) Thread-1: in serve(), blocked on accept()
(2) Thread-2: in stop(), calls interrupt()
(3) Thread-2: in stop(), in interruptChildren(), checked that socket is valid
(4) Thread-1: in serve(), unblocked because of interrupt(), calls releaseOneDescriptor
(5) Thread-2: in interruptChildren(), send fails so clients are not interrupted