Rather than trying to retrofit an external schema onto the data which has been
loaded.

This is *not* updating the event contents to the newer v2.0 schema: this is
exactly the same data, but rewriting the stored data so that the corresponding
schema is bit-for-bit identical with what we're currently calling v1.0.
(Exactly what generated these is lost in the mists of time...)

This (temporarily) breaks ``printStream.py``, ``filterStream.py`` and other
code which relies on AlertConsumer.

Note that his removes a bunch of unexplained special cases in poll() and
decode_message(). It's not clear if they were left over experiemnts or working
around weird failure modes. Things seem to be working fine without them for
now...

In particular, mentioning the Confluent wire format and providing brief
instructions for unpacking.

This means we have to specify the schema to be used to serialize the data
every time we send a new message. That's arguably clunky, but it makes it
easier to “pass through” the schema from received message to filtered stream.