resourceloader: Support hashes as cache invalidation trigger

For now ResourceLoader will still internally deal with timestamps,
especially because they are easier to max() on the client side
(on the server we could make a hash of different hashes, on the
client that's a bit annoying).

However ResourceLoaderModule#getHashMtime will abstract the logic
we already use in ResourceLoaderLanguageDataModule and have
encouraged others to use, which is to put the timestamp and
hash in Memcached, and use that to observer the hash change and
update the timestamp when it does.

Updated ResourceLoaderLanguageDataModule to make use of this.

Change-Id: Ib051ef41ba239084671c30fd275b8d94099d5d52