Enforce key length constraint of 2048 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.
Bigtable eventually lifted the whole-row size limit once they eliminated the cases that needed to load an entire row into memory. However, they also had a limit on the key size (64KB? most applications self-limited to a few hundred bytes) because the BT servers had to keep lots of keys in memory for various reasons. 

I think it's good to have a clearly-defined limit instead of just "wherever rocksdb breaks", but 2KB seems too low. In particular, it's often useful to use URLs as a component of your keys and urls are allowed to be up to 2KB (and I think that limit is increasing as older browsers age out). I'd make the limit at least 16KB.