Add time validation to health check

The health check code does not have a notion of "time flow": therefore,
two consecutive calls to lttng_health_check() might end up returning a
bad state (0) just because there was too little time between the
invocations.

Add some time information to the "last" snapshot, so we can do a time
delta between the current and last snapshot to figure out if we need to
report the thread as stalled or not.

At this point, a thread is considered stalled with a wait time of over
20 seconds.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: David Goulet <dgoulet@efficios.com>