[WIP]Performance[mqbi::DispatcherEvent]: replace multiple inheritance with variant

[678098]

Before

sizeof(mqbi::DispatcherEvent): 880
mqbi::DispatcherEvent::reset():
       total: 708.62 ms (100000000 iterations)
    per call: 7 ns

After

sizeof(mqbi::DispatcherEvent): 320
sizeof(mqbi::DispatcherPutEvent): 128
sizeof(mqbi::DispatcherPushEvent): 288
sizeof(mqbi::DispatcherDispatcherEvent): 152
sizeof(mqbi::DispatcherCallbackEvent): 80
sizeof(mqbi::DispatcherReceiptEvent): 32
sizeof(mqbi::DispatcherAckEvent): 80
sizeof(mqbi::DispatcherControlMessageEvent): 192
mqbi::DispatcherEvent::reset():
       total: 51.14 ms (100000000 iterations)
    per call: 0 ns

[hallfox]

Do we even need to set this type anymore?

[hallfox]

Suggested change

	bsl::shared_ptr<bdlbb::Blob> d_blob_sp;
	// Blob of data embedded in this
	// event. Refer to the corresponding
	// accessor on the various
	// DispatcherEvent view interfaces
	// for more specific information.
	/// Blob of data embedded in this
	/// event. Refer to the corresponding
	/// accessor on the various
	/// DispatcherEvent view interfaces
	/// for more specific information.
	bsl::shared_ptr<bdlbb::Blob> d_blob_sp;

Docstring style

[hallfox]

Do we need virtual destructors still?

[678098]

No, will clean up this

[hallfox]

Is the refactoring here to make asPushEvent and family to have a const and non-const overload?

[hallfox]

Little wary of this const cast, what's the reason this is okay?

[678098]

Note that initially we are building DispatcherEvent, and after makePushEvent() we are calling setters for DispatcherPushEvent. So we switch between objects amidst these setters calls. This interface is not perfect, but I have plans to replace it by calling constructors with all args instead.

So instead of this

(*dispEvent)
        .setSource(this)  // DispatcherEvent
        .makePushEvent()  // !!! DispatcherPushEvent
        .setBlob(appData) // DispatcherPushEvent
        .setOptions(options) // DispatcherPushEvent
        .setGuid(msgGUID) // DispatcherPushEvent

We will have smth like this:

(*dispEvent)
        .setSource(this)
        .makePushEvent(appData, options, msgGUID, ...)

I plan to do it in a separate PR, with performance benchmarks, to limit the number of changes in this PR.