Compress pendingdata

Summary:
pendingdata can potentially be rather large. Because all entries are of
the form $collectionId:$uid, we also have a lot of duplication in the
string.

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).

Differential Revision: https://git.kolab.org/D2062