on some units it can take around 250ms to reply to a request, by which
time we're shoving another command to the unit. if this happens, the
unit gives up and starts replying to the new command, which can again
take 250ms. in this situation effectively nothing gets through.
avoid this by checking if we're in the parser state machine. this also
gives us timeout handling.
tested on 4 different AC units. one which was unusable before is now
functioning as expected, and the other 3 appear just as functional as
they were before.
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