MB-21617: Change CAS resolution to nanoseconds

MB-21617: Change CAS resolution to nanoseconds

This gives better compatibility with older datasets and a less
confusing experience if an old cluster did set_with_cas against
a new one.

The CAS is generated as a nanosecond value.

Drift is tracked though in microseconds to give a longer window before
we may overflow the counter. The config thresholds are also all µs
but converted to ns so we can correctly track drift.

A new max_cas_str is also added to assist the supportability of
LWW, so at a glance you can see what the max_cas means as a
date/time string.

Change-Id: I40fb89add968043aca01b1de103f62319d814a5c
Reviewed-on: http://review.couchbase.org/69708
Reviewed-by: Dave Rigby <daver@couchbase.com>
Tested-by: buildbot <build@couchbase.com>