Allow more than 256 logging vars/parameters

[whoenig]

This addresses the firmware side of issue #313 by adding additional
CRTP commands (called V2) that uses 16 instead of 8bits for indexing.

This change keeps the old API the same, but caps the number of vars
to 255 to allow older clients to keep working. Eventually, the old
API should be removed.

This change reduced the possible length of variables by 1 byte,
which affected two variables whose name had to be shortened.

Tested with crazyflie_cpp commit 3eb3203. Other clients will need
similar changes to use more than the first 255 variables.

[whoenig]

This change intentionally has lots of code duplications, as I hope that we can simply remove the old API in a future change once the major clients have switched.
I did implement a "hybrid" mode in crazyflie_cpp, that first checks if V2 is available and falls back to V1 if not. This turns out to be pretty tricky, because there is no version information in the firmware right now, and it requires lots of code duplication (for C++ typesafety).

I tried to add similar support to crazyflie-lib-python, but couldn't get the dynamic check to work (there doesn't seem to be a timeout there anymore...). If you decide to simply require newer firmwares, the change will be straightforward.

[ataffanel]

@whoenig thanks a lot for the PR, this is a long needed functionality!

There actually is a protocol version in the Crazyflie, I added a commit to bump this version from 3 to 4, this will allow any client to discover that the new log commands are available.

With that it looks good to me so I am merging the PR and it closes #313.

[whoenig]

Thanks @ataffanel! I added a new issue for crazyflie_cpp to use the protocol version to distinguish old/new firmwares, see whoenig/crazyflie_cpp#6.