cache: Add in-process caching to MessageBlobStore

cache: Add in-process caching to MessageBlobStore

This allows us to continue to use MessageBlobStore::get() as a way
to pre-fetch messages in batches for multiple modules while also
allowing individual modules to retrieve  their messages in a convenient
way.

This is in preparation for implementing ResourceLoaderModule::buildContents
and using it in ResourceLoaderModule::getVersionHash.

ResourceLoader::respond() already fetches message blobs in batches for the
actual response, but it also needs getVersionHash(), which would otherwise
fetch the same messages a second time.

Change-Id: I7e4c8b65765b54807123e85cfbb7eb2e5b2f39bd