Compact style mutation (#464)

* Don't store the full style attribute change, as small mutations to single style properties result in storage of a rewrite for the full style attribute, which may be very large.

Had an example of a website using http://schillmania.com/projects/snowstorm/ where many direct style changes were happening every second across many 'snowflake' elements, with each attribute change looking like:
"style":"color: rgb(255, 255, 255); position: absolute; width: 8px; height: 8px; font-family: arial, verdana; overflow: hidden; font-weight: normal; z-index: 0; display: block; bottom: auto; opacity: 1; padding: 0px; margin: 0px; font-size: 10px; line-height: 10px; text-align: center; vertical-align: baseline; left: 242.807px; top: 85.7332px;"
even though maybe just the left/top position had been changed

* More compact storage for the much more common attribute value without an `!important` flag - saves 6 chars per style attr in the json :)

* Fix bug: attributes weren't getting removed after changes to treatment of 'style' attributes

test/integration.test.ts

@@ -184,7 +184,8 @@ describe('record integration tests', function (this: ISuite) {
 
     // toggle the select box
     await page.click('.select2-container', { clickCount: 2, delay: 100 });
-
+    // test storage of !important style
+    await page.evaluate('document.getElementById("select2-drop").setAttribute("style", document.getElementById("select2-drop").style.cssText + "color:black !important")');
     const snapshots = await page.evaluate('window.snapshots');
     assertSnapshot(snapshots, __filename, 'select2');
   });