pendingdata can potentially be rather large. Because all entries are of
the form $collectionId:$uid, we also have a lot of duplication in the
Large entries are problematic beause with ~50k messages we already reach
the mysql max_allowed_packet size of 1MB. Simply compressing that string
imrpoves the situatin drastically.
We allow decompression to fail for backwards comaptiblity (works because
of decompression checks for the relevant headers).