Phase one of the new gnx code:
- replace post-pass with ni.allocateHeldGnxs.
- ni.getNewIndex adds v to hold_set if hold_flag is True.
- Added mandatory v argument to ni.getNewIndex.

Leo build: 20141022143105
Replaced the post-pass by a smarter gnx-allocation scheme.  All tests pass, including a new unit test.
- added gnx keyword argument to VNode ctor.
- Various methods (in leoAtFile.py, leoCache.py, leoCommands.py,  )use this arg to suppress unnecessary calls to ni.getNewIndex.
- Added ni.begin_holding and ni.end_holding.
- ni.getNewIndex now queues vnodes if ni.hold_flag is True.
- Revised tracing in leoPersistence.py

Leo build: 20141022180807