time-util: introduce deserialize_timestamp_value()

time-util: introduce deserialize_timestamp_value()

The time-util.c provides dual_timestamp_deserialize() function to
convert value to usec_t and set it as value of ts->monotonic and
ts->realtime.

There are some places in code which do the same but only for one
clockid_t (realtime or monotonic), when dual_timestamp_deserialize()
sets value of both.

This patch introduces the deserialize_timestamp_value() which converts
 a given value to usec_t and write it to a given timestamp.