l10nupdate: Use refreshMessageBlobs.php instead of clearMessageBlobs.php

The clearMessageBlobs.php script is a bit too heavy-handed: it
obliterates the entire ResourceLoader message caches for all wikis,
which can lead to a database stampede as user requests try to repopulate
the cache all at once.

refreshMessageBlobs.php takes a more nuanced approach: It iterates over
all the cache entries and updates only those where the message cdb has
changed. And it does this one resource at a time, rather than having
every client connection try to do it in parallel.

The downside is that this is a good bit slower.

Note this requires I3b6ae128 be deployed to all active versions.

Bug: 27320
Change-Id: Ic633a7fde8d4a1d9e9326aa5ae52bf1227e8d30f