time-util: add triple timestamp object

time-util: add triple timestamp object

We already have a double timestamp object that we use whenever we need both a
MONOTONIC and a REALTIME timestamp taken and stored. With this change we
also add a triple timestamp object that in addition stores a BOOTTIME
timestamp, which is useful for a few usecases.

Note that we keep dual_timestamp around, as it is useful in many cases where
triple_timestamp is not, in particular because retrieving the monotonic and
realtime timestamps is much cheaper on Linux that getting the boottime
timestamp.