[changemode demo] Really rate-limit looksLikeScheme() calls

After the fix: http://rawgithub.com/cben/CodeMirror/patch-changemode/demo/changemode.html
Type `;var x;` at the start and rapidly delete and re-insert the leading `;` to see the mode switch only after inactivity.

Hard to observe a performance win, but this was clearly the intent of the example code.