MB-20079: Use std::chrono::steady_clock (ProcessClock)

MB-20079: Use std::chrono::steady_clock (ProcessClock)

Change task scheduling to use ProcessClock which is not
affected by changes to wall clock time.

Change-Id: I2fc9688abb782fe2c9e80efb6da840be3643d4a5
Reviewed-on: http://review.couchbase.org/69899
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Dave Rigby <daver@couchbase.com>
Reviewed-by: Jim Walker <jim@couchbase.com>