Minor optimization to the AutoLoader

Minor optimization to the AutoLoader

When MediaWiki autoloading fails, we should gracefully return false.
Instead, we have been calling strtolower roughly 1,000 times in the hope
of finding a case-insensitive match.

This patch preserves the legacy case-insensitivity, but improves its
performance by approximately 100x, by storing the case-insensitive class
lookups as a static variable.

There is a new global $wgAutoloadAttemptLowercase which will switch the
behavior if desired.  The default is to support case-insensitive loading.

Change-Id: Ifb12e05614a48390b730167e9d4ddcd8545db764