clock: add monotonic uptime clock

This patch starts to solve both issue #142 ("Support MONOTONIC_CLOCK")
and issue #81 (use <chrono> for time).

First, it adds an uptime() function to the "clock" interface, and
implements it for kvm/xen/hpet by returning the system time from which
we subtract the system time at boot (but not adding any correction
for wallclock).

Second, it adds a new std::chrono-based interface to this clock, in
a new header file <osv/clock.hh>. Instead of the old-style
clock::get()->uptime(), one should prefer osv::clock::uptime::now().
This returns a std::chrono::time_point which is type-safe, in the
sense that: 1. It knows what its epoch is (i.e., that it belongs to
osv::clock::uptime), and 2. It knows what its units are (nanoseconds).
This allows the compiler to prevent a user from confusing measurements
from this clock with those from other clocks, or making mistakes in
its units.

Third, this patch implements clock_gettime(MONOTONIC_CLOCK), using
the new osv::clock::uptime::now().

Note that though the new osv::clock::uptime is almost identical to
std::chrono::steady_clock, they should not be confused. The former is
actually OSv's implementation of the latter: steady_clock is implemented
by the C++11 standard library using the Posix clock_gettime, and that
is implemented (in this patch) using osv::clock::uptime.

With this patch, we're *not* done with either issues #142 or #81.
For issue #142, i.e., for supporting MONOTONIC_CLOCK in timerfd, we
need OSv's timers to work on uptime(), not on clock::get()->time().
For issue #81, we should add a osv::clock::wall type too (similar to
what clock::get()->time() does today, but more correctly), and use either
osv::clock::wall or osv::clock::uptime everywhere that
clock::get()->time() is currently used in the code.
clock::get()->time() should be removed.

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>