resourceloader: Use MD4 to compute file hash rather than SHA1

resourceloader: Use MD4 to compute file hash rather than SHA1

The hash value generated by ResourceLoaderModule::safeFileHash() is used for
versioning and cache invalidation, so a cryptographic hash function is not
necessary. We can get better performance by using MD4.

Wikimedia on-CPU time over 24 hours of safeFileHash() in load.php:
* 6.68% with safeFileHash using SHA1 (day 1)
* 7.07% with safeFileHash using SHA1 (day 2)
* 2.84% with safeFileHash using MD4 (day 3)

Change-Id: I6ff728f1240268517c0f03e0823129316bc901cb