(#3441) don't un-parse ExprTrees when inserting them into the classad cache
when the pre-parsed form is available.  this is performance optimization for classad caching.
that changes is it from slower with caching on to faster with caching on.
This commit also skips unnecessary copyies of std::strings for platforms (win32)
where std::string is not copy-on-write.
===VersionHistory:Pending===
Merge branch 'master' of ingwe:/p/condor/repository/CONDOR_SRC