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.
The MySQL key length is limited by the btree page size. RocksDB has no such key length limitation other than what will fit in memory. I agree that shorter keys will be faster, but that's an application concern and and shouldn't exclude the usage of longer keys unless there is some other restricting factor. For example, cockroach might want to limit the length of a range (in order to limit the time to rebalance/recover a range) which necessarily limits the length of a row and thus the length of key.