refcounter.py: prevent use of excessive RAM with large refs logs

refcounter.py: prevent use of excessive RAM with large refs logs

When processing a 212MB refs file, refcounter.py used over 3GB of RAM.
This change greatly reduces memory usage in two ways:

* Saving object history in whole lines instead of separated values.
* Not saving normal/skewed/leaked object lists unless they are requested.

ASTERISK-23921 #close
Reported by: Corey Farrell
Review: https://reviewboard.asterisk.org/r/3668/

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.8@417480 65c4cc65-6c06-0410-ace0-fbb531ad65f3