The compression is completely transparent to clients, serializers and
Akonadi. The idea is that when serializing payload, we can compress
the serialized data using LZMA compression to save space. The data are
usually large enough to benefit from the compression and at the same
time small enough for the compression to not cause any significant
performance overhead.

In my local experiment, compressing each file in file_db_data reduced
the overall size by ~30%.

The only place where the compression aspects "leak" to the user is
regarding the Item and part sizes stored in Akonadi.

The change is backwards compatible, so it can handle uncompressed
payloads created before this change just fine. All newly created
or updated payloads will get compressed automatically. Eventually
a StorageJanitor task to compress th entire storage may be introduced,
but we may need some proper progress reporting for that, since it
may take a lot of time, even on fast SSD disk to compress all the
files in file_db_data (depending on size of the database).