This commit adds a new feature to the server to add comments at the database
level. 1024 bytes is the maximum comment length allowed. If the comment length
exceeds this limit, a new error/warning code 4144 is thrown, based on whether
thd->is_strict_mode() is true/false. The database comment is also added to the
db.opt file, as well as to the information_schema.schemata table.