revert my changes to convert fields of zt_pvt to use stringfields. When I did

revert my changes to convert fields of zt_pvt to use stringfields.  When I did
this, I was not keeping in mind the fact that after a stringfield is overwritten
by another string, the memory used by the old string can not be recovered.  I
would like to go back through these changes and make sure that stringfields are
not used for fields that are written to many times before these changes are
committed.

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@32759 65c4cc65-6c06-0410-ace0-fbb531ad65f3