Move getHLCCas directly to use std::chrono and static_assert that clock !steady

LWW requires a HLC timestamp and the real-time part of the stamp to be high-resolution and to follow system time and any NTP changes to system time.

We can now use std::chrono directly from ep-engine and no longer need to rely on the gethrtime abstraction which can compile to different things (cxxtime.cc vs gethrtime.cc).

If we use std::chrono directly we can also clearly use static_assert which can check the is_steady bool for false (we must not have a steady_clock for LWWs use of the HLC cas).

Note the current code on centos6 and centos7 appears to be safe, in that running a test against our gethrtime function it follows system-time on both platforms. A full audit of all Linux platforms is slow and this MB covers a sensible change which will help us.