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