An EventSource whose request never gets response headers stays in
CONNECTING without firing open or error, so the backoff never runs and
the staleness watchdog (which only judges a 'connected' stream) cannot
see it. Bound each attempt with a 15s connect timeout: if open has not
fired, close the attempt and retry through the same path onerror uses.
The timer is cleared on open, on failure and at the top of every
connectSSE(), and both the timeout and the failure handler ignore an
EventSource that has since been replaced.
Aamer Akhter
536eca8f3d