First attempt at limiting key size

This commit adds a check on every write command, that will err the command if
any of the keys is longer than some arbitrary limit, if this is deemed not
terrible, I'll follow up with a commit that makes the max key size a
configurable parameter, and improve the err message.

When the param is configurable the check can be done only when configured as well.

@fbogsany @csfrancis 
:+1: I prefer the complex initializer. With the current version, you don't need to initialize `keys` to `NULL`.