Exec() now supports reporting results for multiple statements in a request (#1261)

[mherr-google]

Description

Exec() now provides access to the last inserted ID and number of affected rows for all statements, not just the last one. This is useful to execute batches of statements such as UPDATE with minimal roundtrips. Fixes #1261.

The approach taken is to track last insert id and affected rows using []int64 instead of a int64. Both are set in mysqlResult, and a new mysql.Result interface makes them accessible to callers calling Exec() via sql.Conn.Raw. For example:

conn.Raw(func(conn interface{}) error {
        ex := conn.(driver.Execer)
        res, err := ex.Exec(`
        UPDATE point SET x = 1 WHERE y = 2;
        UPDATE point SET x = 2 WHERE y = 3;
        `, nil)
        // Both slices have 2 elements.
        log.Print(res.(mysql.Result).AllRowsAffected())
        log.Print(res.(mysql.Result).AllLastInsertIds())
      })

Checklist

• Code compiles correctly
• Created tests which fail without the change (if possible)
• All tests passing
• Extended the README / documentation, if necessary
• Added myself / the copyright holder to the AUTHORS file

[aforrest-google]

There is a memory loitering issue associated with this change. Details in this closed issue: github.com//issues/1304.

I have a fix that should join this PR before it is merged. mherr-google@ please contact me.

[mherr-google]

The memory loitering issue described in #1304 is fixed, this is now ready for review.

[mherr-google]

This is superceded by #1309, which is the final version of the change being used internally (revised to address memory loitering).