WL#6984: Deprecate ENCODE()/DECODE() in 5.7

Since MySQL has the much more advanced AES_ENCRYPT()/AES_DECRYPT() we should
move on and deprecate ENCODE()/DECODE() in favor of the AES variants.

User Documentation
==================

http://dev.mysql.com/doc/refman/5.7/en/encryption-functions.html

In mysql-trunk any execution of the ENCODE() or DECODE() function will trigger 
the following warning : 
Code : ER_WARN_DEPRECATED_SYNTAX (1287 currently)
Message for encode : 'ENCODE' is deprecated and will be removed in a future 
release. Please use AES_ENCRYPT instead
Message for decode : 'DECODE' is deprecated and will be removed in a future 
release. Please use AES_DECRYPT instead