* Document the structure of the in-process $titleInfo cache.
Specifically, specify that it is not the value from getTitleInfo(),
but rather a container for zero or more versions of such values.
The reason this is fragmented is because ResourceLoaderContext
is a parameter to most methods and as such, makes everything
variable. Tracked as T99107.
* Make various bits easier to understand by consistently refering
to the container keys as "batchKey", and referring to the internal
keys as "titleKey".
* Centralise title key logic by moving to private method.
* Replace the internal creation of titleKey to be based on LinkTarget
with plain namespace IDs and db keys, instead of invoking the
expensive getPrefixedTitle function which involves quite a lot
of overhead (TitleCodec, GenderCache, Database, Language,
LocalisationCache, ..).
Change-Id: I701e5156ef7815a0e36caefae5871524eff3f688
dfb754cbc6