database/sql: prevent race on Rows close with Tx Rollback

In addition to adding a guard to the Rows close, add a var
in the fakeConn that gets read and written to on each
operation, simulating writing or reading from the server.

TestConcurrency/TxStmt* tests have been commented out
as they now fail after checking for races on the fakeConn.
See issue #20646 for more information.

Fixes #20622

Change-Id: I80b36ea33d776e5b4968be1683ff8c61728ee1ea
Reviewed-on: https://go-review.googlesource.com/45275
Run-TryBot: Daniel Theophanes <kardianos@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/database/sql/sql.go

@@ -2700,7 +2700,9 @@ func (rs *Rows) close(err error) error {
 		rs.lasterr = err
 	}
 
-	err = rs.rowsi.Close()
+	withLock(rs.dc, func() {
+		err = rs.rowsi.Close()
+	})
 	if fn := rowsCloseHook(); fn != nil {
 		fn(rs, &err)
 	}