[WIP] Add Async http appender

[iwillspeak]

Creates a new AsyncGelfHttpAppender which performs logging in the background as the other Async* appenders do.

This PR also makes a few refactorings to share code more amongst the async appenders:

• BufferedLogSender is added. This is responsible for queing and sending messages in the background
• Switch to Task for background processing. Uses async operations when sending data.
• Move to BlockingCollection for buffer of lines to send. This should be faster in this producer-consumer scenario.
• Move to CancellationTokenSource and CancellationToken for cancellation of the background processing.

Still TODO:

• [x] Choose a Task count based on a parameter rather than hardcoding it
• [x] Choose a buffer size based on a parameter rather than hardcoding.
• [ ] Add docs to README for new appender
• [x] Add type forwarding for new appender

[jjchiw]

Wow, finally :)

Thanks for the PR

[iwillspeak]

Thanks for this!