Implement isc_interval_t on top of isc_time_t

Implement isc_interval_t on top of isc_time_t

Change the isc_interval_t implementation from separate data type and
separate implementation to be shim implementation on top of isc_time_t.
The distinction between isc_interval_t and isc_time_t has been kept
because they are semantically different - isc_interval_t is relative and
isc_time_t is absolute, but this allows isc_time_t and isc_interval_t to
be freely interchangeable, f.e. this:

    isc_time_t *t1;
    isc_interval_t *interval;
    isc_time_t *t2;

    isc_interval_set(interval, isc_time_seconds(t2), isc_time_nanoseconds(t2);;
    isc_time_subtract(t1, interval, t2);
    isc_interval_set(interval, isc_time_seconds(t2), isc_time_nanoseconds(t2));

to just:

    isc_time_t *t1;
    isc_interval_t *interval;
    isc_time_t *t2;

    isc_time_subtract(t1, t2, interval);

without introducing a whole set of new functions.