Garbage Collector thread can block forever if system time is set back

[jeandube]

Problem discovered on Linux aarch64 (was not present on armv7l). Thanks to @StephaneStH for diagnostic and fix for this problem. Our firmware monitors all threads and asserts if anyone of them stops looping. The problem is reproduced with a SRT listener in listening state while system time is set in the past: the pthread_cond_timedwait() scheduled to timeout every 1 sec. will never timeout. This is probably not critical to most systems but it shows a design flaw. The fix that will be submitted in a pull request is to use clock_gettime(CLOCK_MONOTONIC, &timeout); instead of gettimeofday(&now,0).