Rewrote r69339 etc. to clean up API cache header handling.

Rewrote r69339 etc. to clean up API cache header handling.

* Introduced a "cache mode" concept to simplify the header generation code, and to avoid odd results when conflicting cache header requests are received from submodules, or at least to formalise the handling of such cases.
* Made the cache mode private by default, so that code written in ignorance of caching tends to be safe. If different query modules are used in a single request, private caching is preferred over public caching.
* Removed the "must-revalidate" option from all CC headers, this is really specific to page views with a hacked squid in front, I don't think it's applicable here.
* Made the watchlist module private. This is really the definition of private data. There's nothing in the HTTP spec that says the URL for a CC:public request is private and can't be leaked. CC:private provides protection against unknown proxy behaviour.
* In ApiQueryAllmessages: avoid calling $wgLang->getCode() to check if it's necessary to make a new $wgLang when lang= is specified, since this is the only thing that unstubs $wgUser.
* Removed "FIXME: should this check $user instead of $wgUser?" Answer is no.