Implement reading buffers in term of ReadBuf

[notgull]

RFC 2930 introduces a safe reading buffer of type ReadBuf. Once this reaches fruition, I'd like to replace the current buffer-zeroing implementation with a ReadBuf.

[notgull]

Note that buffer reading uses a single buffer that's reused between connections (actually, two buffers, but who's counting?). This issue doesn't really matter anymore.