Move away from BufferQueue as basic element for everything

[Geal]

the BufferQueue implementation is very specific to the HTTP 1 proxy, awkward to use in the Pipe protocol, and probably unusable for HTTP/2.

I propose the following steps:

• use a pool of Buffer instead of BufferQueue, and use that pool to initialize a BufferQueue
• use Buffer directly in protocols like Pipe
• move BufferQueue specific details out of the HTTP 1 parser
• use the Buffer pool in the HTTP/2 parser

side note: should we move to the bytes crate instead?