MINOR: http: replace http_message_realign() with buffer_slow_realign()

MINOR: http: replace http_message_realign() with  buffer_slow_realign()

There is no more reason for the realign function being HTTP specific,
it only operates on a buffer now. Let's move it to buffers.c instead.

It's likely that buffer_bounce_realign is broken (not used), this will
have to be inspected. The function is worth rewriting as it can be
cheaper than buffer_slow_realign() to realign large wrapping buffers.