Limit key size unstable

same as #1968 but PRing against unstable.

## Problem

I've observed redis servers eat up all the machines memory and get into swapping causing issues on the clients, when a bug or just careless usage introduces very large keys (100s of MBs).

## What is here?

This adds the ```max-key-size``` config option, when the value of that option is not null keys longer than the number in bytes defined will be rejected.

I'm PRing against 2.8, I can move it to 3.x if there is interest.

Topic on the google group: https://groups.google.com/forum/#!topic/redis-db/aLxpHURnA8Q

/cc @csfrancis, @fbogsany

what do you think ? @antirez @pietern @badboy