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.
Addressed Tobias feedback, increased key size limit to 4096
Merge pull request #106 from cockroachdb/spencerkimball/enforce-key-length

Enforce key length constraint of 4096 bytes.