Buffer data before sending it to the compressors

[CAFxX]

Compressors generally operate faster if they are provided with large chunks of data.

There is already a small buffer, but it's only used for content-type sniffing before compression start (and is bypassed entirely once compression has started).

We could instead replace it with a buffer of configurable size (by default larger than the current content-type sniffing one) that would be used also during compression to compress data in larger chunks.

Some compressors may do the same internally, so we would need to avoid double buffering in this case.

Such a buffer could later also be used to capture the full uncompressed payload of (small) responses, that paired together with the resulting compressed payload could be cached to avoid having to recompress the same data over and over.