Fewer memory allocations

Feel free to take all or part of this.  I'll be away until Monday, at which time I can resume working on memory/CPU/contention in the MySQL driver and database/sql code.

Before: 603 B/op	      24 allocs/op
After: 344 B/op	      15 allocs/op

Fewer garbage collection events, since we're allocating less.

From:

```go
package main

import (
        "database/sql"
        "sync"
        "sync/atomic"
        "testing"

        _ "github.com/go-sql-driver/mysql"
)

const concurrencyLevel = 10

type TB interface {
        Fatal(...interface{})
}

func check(tb TB, err error) {
        if err != nil {
                tb.Fatal(err)
        }
}

func initDB(tb TB) *sql.DB {
        checkRes := func(res sql.Result, err error) {
                check(tb, err)
        }
        db, err := sql.Open("mysql", "root:r@/test?charset=utf8")
        check(tb, err)
        db.SetMaxIdleConns(concurrencyLevel)
        checkRes(db.Exec("DROP TABLE IF EXISTS foo"))
        checkRes(db.Exec("CREATE TABLE foo (id INT PRIMARY KEY, val CHAR(50))"))
        checkRes(db.Exec(`INSERT INTO foo VALUES (1, "one")`))
        checkRes(db.Exec(`INSERT INTO foo VALUES (2, "two")`))
        return db
}

func BenchmarkQuery(b *testing.B) {
        b.StopTimer()
        b.ReportAllocs()
        db := initDB(b)
        defer db.Close()

        stmt, err := db.Prepare("SELECT val FROM foo WHERE id=?")
        check(b, err)
        defer stmt.Close()
        b.StartTimer()
        
        remain := int64(b.N)
        var wg sync.WaitGroup
        wg.Add(concurrencyLevel)
        defer wg.Wait()
        for i := 0; i < concurrencyLevel; i ++ {
                go func() {
                        defer wg.Done()
                        for {
                                if atomic.AddInt64(&remain, -1) < 0 {
                                        return
                                }
                                var got string
                                check(b, stmt.QueryRow(1).Scan(&got))
                                if got != "one" {
                                        b.Errorf("query = %q; want one", got)
                                        return
                                }
                        }
                }()
        }
}
```