Add "rolling compression" to file writer and encodings

[westonpace]

The compression in 2.1 currently has a problem. We accumulate 8MB of raw data before we run compression. Then we compress the buffer and write a page. However, when the data is highly compressible, this could lead to a very small page being created. Instead of writing a page at that point we should instead wait and add more data.

This will require enhancements to all encoders as it will require them to be able to "save off state" and "resume with more data".

[westonpace]

Since this is a writer-only feature, it could be added forwards & backwards compatible

[Xuanwo]

Should we include this in version 2.1 or leave it for a future 2.2 release?

[westonpace]

We should defer to a future release. I'll update title.