[hack] change event store uuid type to string.

The DBAL event store uses `guid` as its type for event uuids (which
makes sense). Unfortunately, we use string IDs in our tests for
convenience (like "offer_1"). The type should be configurable, but
we'll just use `string` for the time being.
Merge pull request #3 from lrqdo/event_store_uuids

[hack] change event store uuid type to string.