ZFS file sync overhaul for API syncing

This mostly gets ZFS file syncing and file conflict resolution working
with the API sync process. WebDAV will need to be updated separately.

Known issues:

- File sync progress is temporarily gone
- File uploads can result in an unnecessary 412 loop on the next data
  sync
- This causes Firefox to crash on one of my computers during tests,
  which would be easier to debug if it produced a crash log.

Also:

- Adds httpd.js for use in tests when FakeXMLHttpRequest can't be used
  (e.g., saveURI()).
- Adds some additional test data files for attachment tests

test/tests/syncEngineTest.js

@@ -19,28 +19,20 @@ describe("Zotero.Sync.Data.Engine", function () {
 		var caller = new ConcurrentCaller(1);
 		caller.setLogger(msg => Zotero.debug(msg));
 		caller.stopOnError = true;
-		caller.onError = function (e) {
-			Zotero.logError(e);
-			if (options.onError) {
-				options.onError(e);
-			}
-			if (e.fatal) {
-				caller.stop();
-				throw e;
-			}
-		};
 		
+		Components.utils.import("resource://zotero/config.js");
 		var client = new Zotero.Sync.APIClient({
-			baseURL: baseURL,
+			baseURL,
 			apiVersion: options.apiVersion || ZOTERO_CONFIG.API_VERSION,
-			apiKey: apiKey,
-			concurrentCaller: caller,
+			apiKey,
+			caller,
 			background: options.background || true
 		});
 		
 		var engine = new Zotero.Sync.Data.Engine({
 			apiClient: client,
-			libraryID: options.libraryID || Zotero.Libraries.userLibraryID
+			libraryID: options.libraryID || Zotero.Libraries.userLibraryID,
+			stopOnError: true
 		});
 		
 		return { engine, client, caller };