Reduce memory copies in spdy compression implementation.

Motivation:

Currently we do more memory copies then needed.

Modification:

- Directly use heap buffers to reduce memory copy
- Correctly release buffers to fix buffer leak

Result:

Less memory copies and no leaks
I just thought it would be more consistent to have it passed in the encoder and decoder. Also when we get it from the buffer itself we depend on what was used to allocate this buffer which maybe unpooled for example.