Implement async batching via queue

[augustoproiete]

Currently each log entry is immediately written to the RichTextBox control when writes occur on the main thread, or immediately scheduled to be written to the RichTextBox control in the main UI thread via BeginInvoke otherwise.

This sometimes can cause the UI to become unresponsive when writing a large number of messages at the same time.

We should instead write messages to an in-memory queue that is consumed by a background worker on a separate thread, and implement batching and backpressure controls.

[augustoproiete]

The Notepad sink has a very similar need thus whatever approach is implemented there can probably be copied to this sink and vice-versa.