CONN-430

* added new methods in `WallMessage`: `getETag()`, `invalidateCache()` and `invalidateMessagePageCache()`,
* added invalidating `page_touched` value in different actions in `WallExternalController`,
* added setting `ETag` with `PageOutput::setETag()` in `WallBaseController`,
* added more URLs to the purging list,
* additional clean up: removed unused variables, "back to the roots"...

@macbre, @themech, @wladekb feel free to leave your comments, please :)