ResourceLoaderModule: cache file content hash

ResourceLoaderModule: cache file content hash

Cache the content hash of module files in APC, if available. Include the
modification time in the cached value, so that touching the file invalidates
the cache.

Does this take us back to a world in which updates to a file's mtime cause a
module's version to change? No. The content hash cache will be invalidated,
and the content hash will be computed, but the actual value should remain
the same.

Change-Id: I5ceb8537c3cdb120beae19740635f085ee128272