Add diameter_tcp option throttle_cb

To let a callback module decide whether or to receive another message
from the peer, so that backpressure can be applied when it's
inappropriate. This is to let a callback protect against reading more
than can be processed, which is otherwise possible since diameter_tcp
otherwise always asks for more.

A callback is made after each message, and can answer to continue
reading or to ask again after a timeout. It's each message instead of
each packet partly for simplicity, but also since this should be
sufficiently fine-grained. Per packet would require some interaction
with the fragment timer that flushes partial messages that haven't been
completely received.

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