WebDAV file sync overhaul for 5.0

Also:

- Remove last-sync-time mechanism for both WebDAV and ZFS, since it can
  be determined by storage properties (mtime/md5) in data sync
- Add option to include synced storage properties in item toJSON()
  instead of local file properties
- Set "Fake-Server-Match" header in setHTTPResponse() test support
  function, which can be used for request count assertions -- see
  resetRequestCount() and assertRequestCount() in webdavTest.js
- Allow string (e.g., 'to_download') instead of constant in
  Zotero.Sync.Data.Local.setSyncState()
- Misc storage tweaks

test/tests/zoteroPaneTest.js

@@ -148,9 +148,7 @@ describe("ZoteroPane", function() {
 			// TODO: Test binary data
 			var text = Zotero.Utilities.randomString();
 			yield item.saveTx();
-			yield Zotero.Sync.Storage.Local.setSyncState(
-				item.id, Zotero.Sync.Storage.SYNC_STATE_TO_DOWNLOAD
-			);
+			yield Zotero.Sync.Storage.Local.setSyncState(item.id, "to_download");
 			
 			var mtime = "1441252524000";
 			var md5 = Zotero.Utilities.Internal.md5(text)