Event batching

[MatthewWid]

Add a mechanism that allows the user to batch events emitted with push over time and then emit them all at once.

This will allow users with more advanced use-cases to optimise their code if they are submitting a large number of events in a short period of time, minimising the number of calls to res.write.

This will also be used in the new history API (#16) to batch the mass number of event emissions when sending missed events back to the client.

There are some options for what this API can look like listed below.

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The first is having a stateful tracking of when batching is turned on or off and then disabling the no-oping the dispatch method while it is enabled:

session.beginBatch();
...
session.endBatch();

This is likely the most complex option to implement and may have a negative performance impact as every call to the data-writing methods will now have to check if batching is enabled or not.

---

The second option is to allow the user to provide a callback and have any method calls made within that callback be batched internally and then flushed upon completion:

session.batch(() => {
    ...
    session.push("Hello world");
    ...
});

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The third is adding a new Batched class which stores the original session and provides special versions of all the helper methods (push, stream and iterate). This is accessed via a user-given callback on the Session class.

session.batch((batched) => {
    ...
    batched.push("Hello world");
    ...
});

This is the easiest to implement and likely the most performant, though may be a little confusing when you need to access non-helper methods as Batched won't be a sub-class of Session, so you'll need to use the original session instance instead.