fix character encoding handling and use UTF8MB4_UNICODE_CI by default

[sidorares]

Basically, we assume everything is utf8 because this is what client tells server during connection.
This is not enough, because

• server is able to ignore / override this
• different columns in result set could be using own charsets. Column charset is part of column definition packet

see mysqljs/mysql#1408
and mysqljs/mysql#804

would really like to have this as part of 1.0 because this change is semver-major