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.
It's in the right general range, though composite keys and multi-byte
encodings can get tight.

Mysql / innodb maximums are on about 3000 bytes

http://dev.mysql.com/doc/refman/5.6/en/innodb-restrictions.html