Enforce key length constraint of 4096 bytes.

Aside from needing to enforce a constraint of some sort, this change
allows us to definitively specify KeyMax, removing the previous requirement
that keys stored in the map couldn't start with \xff.
You should put that rationale in the code and note that the particular value is somewhat arbitrary. I agree that having a limit is good (though I'd like to see a larger limit), but the current comment falls in the realm of bad commenting.