BUG/MINOR: checks: Reschedule check on observe mode only if fastinter is set

BUG/MINOR: checks: Reschedule check on observe mode only if fastinter is set

On observe mode, if a server is marked as DOWN, the server's health-check is
rescheduled using the fastinter timeout if the new expiration date is newer
that the current one. But this must only be performed if the fastinter
timeout is defined.

Internally, tick_is_lt() function only checks the date and does not perform any
verification on the provided args. Thus, we must take care of it. However, it is
possible to disable the server health-check by setting its task expiration date
to TICK_ETERNITY.

This patch must be backported as far as 2.2. It is related to