Sort acl_users inside the mysql_rename_user()'s loop, after every
successful iteration.
This is needed because on the next loop's iteration find_user_exact()
is used, which requires correct sorting by name.
Don't distinguish between a "password hash" and "authentication string"
anymore. Now both are stored in mysql.user.authentication_string, both
are handled identically internally. A "password hash" is just how some
particular plugins interpret authentication string.
Set mysql.user.plugin even if there is no password. The server will use
mysql_native_password plugin in these cases, let's make it expicit.
Remove LEX_USER::pwhash.
