Add osv::clock::wall

Add a type-safe std::chrono interface to the wall-clock time. E.g., use
use osv::clock::wall::now() to get an std::chrono::time_point instead
of clock::get()->time() which returns an s64.

This patch also changes clock_gettime(CLOCK_REALTIME) and gettimeofday()
to use the new osv::clock::wall::now() interface.

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>