Performance improvements for System::getProcessTime (now ~30% faster)

- Increase test coverage and add performance check to test_GaudiTiming
- Remove unnecessary calls to getpid (saves ~10ns per query() call):
    - Remove getpid calls in case pid is -1 (self)
    - Remove ugly define that was hiding the getpid() calls in the
      source code
- Switch from getrusage() to times() (saves ~100ns per call)
- Only create ProcessHandle for Windows
- Add templated version of currentTime() so it can be inlined
- Allow System::getProcess to be inlined (move Windows code to
  constructor)
- Remove private caches for query() calls as those were not used

See merge request !228