Revert "Use monotonic clock to manage sofia timer instead of system one"

BC/public/flexisip!720 (merged)

This reverts commit 24e19af6.

Side effects have been observed since the su_timer_t uses the monotonic clock to schedule the tasks. Especially, the function su_timer_set_at() was broken because the user of this function was using wall clock timestamps whereas the implementation was working with the monotonic clock. It may be noted that su_timer_set_at() specification cannot be changed as it is non-sense to specified a date by a monotonic timestamp.