REORG/MAJOR: use "struct channel" instead of "struct buffer"

REORG/MAJOR: use "struct channel" instead of "struct buffer"

This is a massive rename. We'll then split channel and buffer.

This change needs a lot of cleanups. At many locations, the parameter
or variable is still called "buf" which will become ambiguous. Also,
the "struct channel" is still defined in buffers.h.