WL#8186: Deprecate conversion of pre MySQL 5.1 encoded database names

Quoting from the manual (http://dev.mysql.com/doc/refman/5.7/en/alter-database.html)
"For example, if a database in MySQL 5.0 has the name a-b-c, the name contains 
instances of the - (dash) character. In MySQL 5.0, the database directory is
also named a-b-c, which is not necessarily safe for all file systems. In MySQL
5.1 and later, the same database name is encoded as a@002db@002dc to produce a
file system-neutral directory name."

Currently we have ALTER DATABASE ... UPGRADE DATA DIRECTORY NAME to do
the conversion to 5.1+ encoding. This is used by mysqlcheck (--fix-db-names
and --fix-table-names) which is again used by mysql_upgrade.

The proposal is to deprecate (5.7) and remove (5.8) this functionality.
We only support upgrade from one major version to the next so that mysql 5.0
encoded names should be gone by now.