scheduler: fix double close of timer_fd (#3058 #3326)
scheduler: fix double close of timer_fd (#3058 #3326)

When a retry is requested, a timer is created and a 'request' for a
'retry' associated to it. Upon 'request' context destroy, the code
is wrongly closing the file descriptor of the timer, and the timer
handler closing it again leading to file descriptor re-use and
further corruptions on the other components of the engine.

Huge thanks to @hsmatulisgoogle @qingling128 @igorpeshansky @quentinmit
@davidbtucker and @agup006 for helping to troubleshooting this
issue!

Signed-off-by: Eduardo Silva <b6525c140147034c280e3cd2f39161f32f3f4f62@gmail.com>

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