OctoPrint (and other software) will sometimes send additional commands
even if the previous command has not yet responded with an "ok".
Change the g-code input reading code to keep reading input so that an
M112 can be detected (and processed out-of-order). To avoid the
extreme case where one writes an entire g-code file to the input
without any pacing, disable the input reading if more than 20 commands
are pending.
Signed-off-by: Kevin O'Connor <kevin@koconnor.net>