vclock: introduce vclock_min/max and vclockset_foreach

This commit introduces new methods for vclock library.

Sometimes we need to take into account the 0-th component of vclock,
as it's done in the following commit, that's why vclock_min/max are
added. vclockset_foreach is just a macros, which allows to iterate
over vclockset comfortably.

NO_DOC=internal
NO_CHANGELOG=internal

(cherry picked from commit 3bf55ab9)