QPID-5865: Be more robust in face of system clock being changed:
- Separate Wall clock time uses from other time
  * (assumed that any time with respect to the epoch is wallclock)
- For Posix use CLOCK_MONOTONIC for all non wall clock purposes
  so that changing system time doesn't affect internal timekeeping
- For Windows kept the same time keeping scheme.

git-svn-id: https://svn.apache.org/repos/asf/qpid/trunk@1607140 13f79535-47bb-0310-9956-ffa450edef68