Port to `System.Threading.Channels`

[nblumhardt]

This is effectively a complete rewrite; it's intended to be more efficient, robust, and modern, but it's going to need some bake time.

The sink now uses System.Threading.Channels to implement an async queue, so there's no longer a blocked worker thread waiting to process batches.

The sink also now begins processing batches as soon as they're full, so if the batch size is 100, and 100 events are written, they'll be processed regardless of whether the buffering delay (PeriodicBatchingSinkOptions.Period) has elapsed.

The sink now properly avoids overlapping batches (the old Monitor-based logic was/is broken, as far as I can tell). Shut-down/flush logic is also cleaner: the queue will always be fully-flushed on shutdown, except when an error occurs.

Finally:

• The internals of the sink are now probably close enough to what would be required by Serilog.Sinks.Async that the two could look at converging in the future (WriteTo.Batched(wt => ...) is also a reasonable thing to consider doing with the new API)
• The internals could now support a concurrency level > 1 by kicking off multiple instances of the run loop, if throughput requires it

I've dropped out the old performance tests, which only effectively covered the custom blocking collection type that the sink used. I'll explore performance using downstream sinks and try to formulate a better perf plan.

Breaking changes:

• TFM support has been narrowed to effectively only targets supported by netstandard2.0; version 3.x of this project can continue supporting older/obsolete TFMs
• The inheritance-based mode is gone; consumers always implement IBatchedLogEventSink and instantiate PeriodicBatchingSink as a wrapper
• Queue size matters more, as it affects shut-down flush time
• Batching logic has changed, so performance may vary in unexpected ways from the older version

[nblumhardt]

Thanks @bartelink! Currently just roughing it out - will loop back and make sure I've sorted those things out once I've decided on how shutdown should work 👍

[nblumhardt]

Tests are failing on .NET Framework (yay 🙄), will sort that out shortly, and then with some luck this will be ready for a look! :-)

[nblumhardt]

I think this is ready for a closer look now :-)

[nblumhardt]

@skomis-mm hope all is well with you! Just flagging this as one that would benefit from your eyes, in case you're able to take a look at any point.

Planning to merge fairly soon, to keep pushing ahead, but expecting to run through a few iterations before calling it "stable".

[nblumhardt]

Thanks for the awesome review @bartelink 👍 should all be done, now.

[nblumhardt]

Published to NuGet now in 4.0.0-dev-*