The resulting code is actually simpler, and more powerful at the
same time

This clarifies the code. It also reduces the code size
by another 1000 bytes or so.

* move the PROTOCOL3_ #defines from .h to .c and turn them into enums
  (debugging enums is easier than #defines)
* add sync factor constants for protocols 1 and 2
* use the PROTOCOL* constants in more places

These help to catch bugs, and also can sometimes help the compiler
optimize the code

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- receive without callback now

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