Handle explicit MySqlDbType for prepared statements more efficiently

[bgrainger]

For #659 the fix in ce16109 ignored all explicit MySqlParameter.MySqlDbType values and just used the inferred type from MySqlParameter.Value.GetType(). This is probably needlessly inefficient, in terms of runtime performance, and number of bytes on the wire (e.g., if an int typed as MySqlDbType.Short could be sent as two bytes, not four).

Investigate if there's an efficient way to respect MySqlParameter.MySqlDbType when serializing binary parameters.