[announce]Max key lenght limit of KV data type

[ideawu]

The SSDB document says "Max key length" is 200 bytes - http://ssdb.io/docs/limit.html no matter what type of data that key is.

But I recently found out that the max key length limit does not applied to KV data type, it was a implementation mistake, it should followed the document.

So, the max key length limit of KV data type will be implemented. This is a incompatible change.

If you ever store any key with length larger that 200 bytes in SSDB with older version, you will not be able to access to that key with later versions.