Reuse the result channel in QueryContexxt

This reduces the number of allocs needed by half.

goos: darwin
goarch: arm64
pkg: github.com/mattn/go-sqlite3
cpu: Apple M1 Max
              │    2.txt    │               1.txt                │
              │   sec/op    │   sec/op     vs base               │
Scan10Cols-10   267.7µ ± 1%   262.7µ ± 2%  -1.86% (p=0.023 n=10)

              │     2.txt     │                1.txt                 │
              │     B/op      │     B/op      vs base                │
Scan10Cols-10   17.057Ki ± 0%   6.070Ki ± 0%  -64.41% (p=0.000 n=10)

              │   2.txt    │               1.txt                │
              │ allocs/op  │ allocs/op   vs base                │
Scan10Cols-10   247.0 ± 0%   138.0 ± 1%  -44.13% (p=0.000 n=10)

sqlite3.go

@@ -451,6 +451,7 @@ type SQLiteRows struct {
 	cls      bool
 	closed   bool
 	ctx      context.Context // no better alternative to pass context into Next() method
+	resultCh chan error
 }
 
 type functionInfo struct {
@@ -2222,23 +2223,33 @@ func (rc *SQLiteRows) Next(dest []driver.Value) error {
 		return io.EOF
 	}
 
-	if rc.ctx.Done() == nil {
+	done := rc.ctx.Done()
+	if done == nil {
 		return rc.nextSyncLocked(dest)
 	}
-	resultCh := make(chan error)
+
+	// Fast check if the Context is closed.
+	if err := rc.ctx.Err(); err != nil {
+		return err
+	}
+	if rc.resultCh == nil {
+		rc.resultCh = make(chan error, 1)
+	}
 	go func() {
-		resultCh <- rc.nextSyncLocked(dest)
+		rc.resultCh <- rc.nextSyncLocked(dest)
 	}()
 	select {
-	case err := <-resultCh:
+	case err := <-rc.resultCh:
 		return err
-	case <-rc.ctx.Done():
+	case <-done:
 		select {
-		case <-resultCh: // no need to interrupt
+		case <-rc.resultCh: // no need to interrupt
 		default:
-			// this is still racy and can be no-op if executed between sqlite3_* calls in nextSyncLocked.
+			// this is still racy and can be no-op if executed between
+			// sqlite3_* calls in nextSyncLocked.
 			C.sqlite3_interrupt(rc.s.c.db)
-			<-resultCh // ensure goroutine completed
+
+			<-rc.resultCh // ensure goroutine completed
 		}
 		return rc.ctx.Err()
 	}