www: start making gzipfilter the parent response class
Virtually all of our responses are going to be gzipped, anyways.
This will allow us to utilize zlib as a buffering layer and
share common code for async blob retrieval responses.
To streamline this and allow GzipFilter to be a parent class,
we'll replace the NoopFilter with a similar CompressNoop class
which emulates the two Compress::Raw::Zlib::Deflate methods we
use.
This drops a bunch of redundant code and will hopefully make
upcoming WwwStream changes easier to reason about.