⚡️ Remove artificial 4ms nextTick() delay when running in a browser

Fixes #646

ShareDB makes heavy use of [`nextTick()`][1]. It notably calls it
every time we [send a message over a `StreamSocket`][2].

If ShareDB is running both `Backend` and `Client` in a browser (eg in
client tests), `nextTick()` will [fall back to `setTimeout()`][3].

However, according to the [HTML standard][4]:

> Timers can be nested; after five such nested timers, however, the
> interval is forced to be at least four milliseconds.

So using `setTimeout()` can incur a penalty of 4ms in the browser, just
idling.

Over the course of a test suite, which makes a lot of fast ShareDB
requests in series, these delays can add up to many seconds or even
minutes of idle CPU time.

This change adds an alternative `nextTick()` implementation, using
`MessageChannel`, which is present in both [Node.js][5] and [HTML][6]
(with slightly different APIs, but close enough for our purposes). This
class offers a way of waiting for a tick on the event loop, without the
arbitrary 4ms delay.

`MessageChannel` is [supported back to even IE10][7], and since  Node.js
v10.5.0, but if for some reason it's missing, we'll still fall back to
`setTimeout()`.

[1]: https://github.com/share/sharedb/blob/5259d0e0b66c50ff9e745fe34a55c5fb16c57a8e/lib/util.js#L88
[2]: https://github.com/share/sharedb/blob/5259d0e0b66c50ff9e745fe34a55c5fb16c57a8e/lib/stream-socket.js#L58
[3]: https://github.com/share/sharedb/blob/5259d0e0b66c50ff9e745fe34a55c5fb16c57a8e/lib/util.js#L98
[4]: https://html.spec.whatwg.org/multipage/timers-and-user-prompts.html#timers
[5]: https://nodejs.org/api/worker_threads.html#class-messagechannel
[6]: https://html.spec.whatwg.org/multipage/web-messaging.html#message-channels
[7]: https://caniuse.com/mdn-api_messagechannel_messagechannel

.mocharc.yml

@@ -2,3 +2,5 @@ reporter: spec
 check-leaks: true
 recursive: true
 file: test/setup.js
+globals:
+- MessageChannel # Set/unset to test nextTick()

lib/util.js

@@ -95,9 +95,20 @@ exports.nextTick = function(callback) {
     args[i - 1] = arguments[i];
   }
 
-  setTimeout(function() {
+  if (typeof MessageChannel === 'undefined') {
+    return setTimeout(triggerCallback);
+  }
+
+  var channel = new MessageChannel();
+  channel.port1.onmessage = function() {
+    triggerCallback();
+    channel.port1.close();
+  };
+  channel.port2.postMessage('');
+
+  function triggerCallback() {
     callback.apply(null, args);
-  });
+  }
 };
 
 exports.clone = function(obj) {

test/util-test.js

@@ -45,6 +45,39 @@ describe('util', function() {
         });
         expect(called).to.be.false;
       });
+
+      describe('without MessageChannel', function() {
+        var _MessageChannel;
+
+        before(function() {
+          _MessageChannel = global.MessageChannel;
+          delete global.MessageChannel;
+        });
+
+        after(function() {
+          global.MessageChannel = _MessageChannel;
+        });
+
+        it('uses a different ponyfill', function(done) {
+          expect(process.nextTick).to.be.undefined;
+
+          util.nextTick(function(arg1, arg2, arg3) {
+            expect(arg1).to.equal('foo');
+            expect(arg2).to.equal(123);
+            expect(arg3).to.be.undefined;
+            done();
+          }, 'foo', 123);
+        });
+
+        it('calls asynchronously', function(done) {
+          var called = false;
+          util.nextTick(function() {
+            called = true;
+            done();
+          });
+          expect(called).to.be.false;
+        });
+      });
     });
   });
 });