Bug #0000268: Code Cleanup in 5.2.x branch

1. created a separate DeleteSectionCache method for "sections_parsed" cache delete, so it no longer is secretly invoked only along with "configs_parsed" cache delete
2. added some explanatory comments to kApplication::Init method
3. unit config cache ("configs_parsed" variable) that was deleted on any configuration variable change, but now it is only deleted when this configuration variable's value is located in "configs_parsed" cache
4. objects of rewrite listeners are now created in separate kRewriteUrlProcessor::rewriteListeners class variable, so it no longer can be accidentally overwritten during unit config cache building process
5. fixed case, when rewrite listener instances could be associated with incorrent unit config prefixes (PHP5 object assignment problem)
6. mod-rewrite url is now successfully processed (using only build-in rewrite listeners) when unit config cache is missing (or needs to be rebuilt) and user is visiting mod-rewrite url
7. "conf_changed" global variable (accessible via GetVar) now contains names of all changes configuration variables

INP-224