Drop the nanotime() function.

Change the few remaining callers to use the appropriate osv::clock or
std::chrono replacements.

We already got rid in previous patches of most references to nanotime()
by switching from absolute times to relative times.

The direct equivalent of the old nanotime() function, where we actually
need the number of nanoseconds since the UNIX epoch, is the rather
verbose expression osv::clock::wall::now().time_since_epoch().count(),
or the shorter clock::get()->time().

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