DDSXTY14 — Add support for data-level compression during serialization

Larga data types or types that contain compressible data (e.g. strings) can benefit from using compression at serialization time.
This would reduce the size required to store the data in the reader/writer caches as well as the bandwidth used to send the data.

This is complementary to transport-level compression and has the advanatage that the data is only compressed once (at serialization, instead of every time it is sent) as well as reducing memory requirements.

Defer to next revision

There is significant complexity in specifying this and there are other urgent issues that need to be addressed so it seems sensible to defer this issue.