Limit amount of buffering

[mistotebe]

Currently, the proxy callbacks make up a naive read/write pair, never limiting the amount of data buffered inside skeeter. This can easily get out of hand with asymmetric connections where one side is able to generate data faster than the other can receive.

Possibly a small state machine interacting with the watermarks should be able to fix this. Not sure about the buffering properties and the verifiability of that code.