vppinfra: fix tw_timer_first_expires_in_ticks for multiple wheels

vppinfra: fix tw_timer_first_expires_in_ticks for multiple wheels

When only the fast in wheel is in use, the next expiring has
to be within the fast_slot_bitmap.

With mutliple wheels, the next expiring timer could be in the
slow wheel. The timers on the slow wheel are only moved into the
fast wheel when the fast wheel index reaches TW_SLOTS_PER_RING.
When calculating the next expiring timer we therefor need to
consider the timers on the slow wheel as well.
When there are no more before reaching TW_SLOTS_PER_RING, instead
of scanning the slow wheel, return the number of ticks until
TW_SLOTS_PER_RING is reached.

Type: fix
Change-Id: I847031f8efc015c888d082f0b0c1bd500aa65704
Signed-off-by: Andreas Schultz <andreas.schultz@travelping.com>
Signed-off-by: Dave Barach <dave@barachs.net>