* In the parser: do link existence tests in batches of 1000. Avoids using excessive...

* In the parser: do link existence tests in batches of 1000. Avoids using excessive memory to store Title objects.
* Split link placeholder/replacement handling into a separate object, LinkHolderArray.
* Remove Title objects from LinkCache, they apparently weren't being used at all. Same unconstrained memory usage as the former $parser->mLinkHolders.
* Introduced ExplodeIterator -- a workalike for explode() which doesn't use a significant amount of memory
* Introduced StringUtils::explode() -- select whether to use the simulated or native explode() depending on how many items there are
* Migrated most instances of explode() in Parser.php to StringUtils::explode()
* Renamed some variables in Parser::doBlockLevels()
* In Parser.php: $fname => __METHOD__, Parser => self/__CLASS__, to support Parser_DiffTest more easily
* Doc update in includes/MessageCache.php for r39412
* MW_TITLECACHE_MAX => Title::CACHE_MAX, nicer name, easier to access from another module