save DB object as self._db for later use by gc_cache_every()

############ deal with this traceback ...:

Finished processing dependencies for zenoss.toolbox==0.1.3
[zenoss@sandbox411master ~]$ zodbscan | tee zodbscan.txt
Traceback (most recent call last):

==================================================

   DATABASE INTEGRITY SCAN:  zodb

==================================================
  File "/opt/zenoss/bin/zodbscan", line 8, in <module>
    load_entry_point('zenoss.toolbox==0.1.3', 'console_scripts', 'zodbscan')()
  File "/opt/zenoss/lib/python2.7/site-packages/zenoss.toolbox-0.1.3-py2.7.egg/zenoss/toolbox/zodbscan.py", line 278, in main
    PKEReporter(db).run()
  File "/opt/zenoss/lib/python2.7/site-packages/zenoss.toolbox-0.1.3-py2.7.egg/zenoss/toolbox/zodbscan.py", line 261, in run
    with gc_cache_every(1000):
  File "/opt/zenoss/lib/python2.7/contextlib.py", line 17, in __enter__
    return self.gen.next()
  File "/opt/zenoss/Products/ZenUtils/AutoGCObjectReader.py", line 86, in gc_cache_every
    db._connectionMap(
AttributeError: 'NoneType' object has no attribute '_connectionMap'