fsevents: fix clever rescheduling

There're could be a situation, where one fsevents handle gets created
and another one is destroyed simultaneously. In such cases
`fsevent_need_reschedule` will be set to 1 twice and reset only once,
leaving handle destructor hanging in uv_sem_wait().

se https://github.com/joyent/node/issues/6296

/cc @bnoordhuis