WIP: 282-Create InFlightNatsMsg to improve channel buffer usage.

[to11mtm]

Here is a Draft PR based on our preliminary discussions in #282 as well as observations I've made, at least as far as how to keep this from being a breaking change or involving too much hard to maintain code (Putting a DU-esque object inside NatsMsg<T> results in a -lot- of painful code, especially if fields get added in the future.)

This does, however, result in a possibly (minorly) breaking change I was not initially aware of; the constructor for NatsJSMsg<T> was made public, and one of the optimizations that was made there was to use _context.Connection (since that way, we can get away with not converting to a message and also make NatsJSMsg<T> smaller).

I would not expect this to break anything internally; however if end users have tests that depend on the behavior of a NatsJSMsg's connection being set to something that isn't what the consumer has... it may cause them problems.

We can always just not take the improvement on NatsJSMsg for now if this is too worrisome.

I'll also note, we -can- also DU NatsMsg<T> as well if we wanted, however for a record struct of that style it is a bit more problematic.

[to11mtm]

Numbers so far are promising... With one exception there is a 5-20% boost on read/write to the channels.

The case where we are suffering badly is ReadAsync where we have to wait on the internal reader; At the moment the custom ChannelReader doesn't have any pooling for it's async operations. I tried to add pooling however... I'm definitely doing something wrong there since alloc numbers didn't change. 😅 Not sure if it's my lack of full understanding of how our pooling works or something I'm doing wrong in the implementation... or if something more thought out is required.

BenchmarkDotNet v0.13.8, Windows 10 (10.0.19045.3803/22H2/2022Update)
Intel Core i7-8750H CPU 2.20GHz (Coffee Lake), 1 CPU, 12 logical and 6 physical cores
.NET SDK 8.0.100
  [Host]   : .NET 6.0.16 (6.0.1623.17311), X64 RyuJIT AVX2
  .NET 8.0 : .NET 8.0.0 (8.0.23.53103), X64 RyuJIT AVX2

Job=.NET 8.0  Runtime=.NET 8.0  

Method	Mean	Error	StdDev	Gen0	Allocated
RunNatsMsgChannel_Sync	61.41 us	0.623 us	0.552 us	-	-
RunInFlightNatsMsgChannel_Sync	55.17 us	0.225 us	0.199 us	-	-
RunNatsMsgChannel_Sync_Pulse	59.99 us	0.315 us	0.263 us	-	-
RunInFlightNatsMsgChannel_Sync_Pulse	54.54 us	0.256 us	0.239 us	-	-
RunNatsMsgChannel_Async_Pulse	87.39 us	0.311 us	0.276 us	-	-
RunInFlightNatsMsgChannel_Async_Pulse	82.23 us	0.368 us	0.326 us	-	-
RunNatsMsgChannel_Async_Pulse_Unhappy	178.55 us	1.279 us	1.196 us	31.2500	147456 B
RunInFlightNatsMsgChannel_Async_Pulse_Unhappy	784.90 us	8.744 us	8.179 us	80.0781	377081 B
RunNatsMsgChannel_Async_Unhappy	119.70 us	1.230 us	1.091 us	0.1221	887 B
RunInFlightNatsMsgChannel_Async_Unhappy	100.23 us	1.514 us	1.416 us	0.2441	1308 B
RunNatsMsgChannel_Async_Happy	117.07 us	0.640 us	0.599 us	-	408 B
RunInFlightNatsMsgChannel_Async_Happy	92.05 us	1.206 us	1.069 us	-	408 B

[to11mtm]

Fixed the pooling issues, however I'm still seeing a huge perf loss in the 'nightmare' case (even though memory usage is now better than the old Reader.) Need to profile to see what's going on.

Very very very close. Just have to improve the 25% dip on unhappy case and I feel like the other regressions will drop off as concerns:

Method	Mean	Error	StdDev	Gen0	Allocated
RunNatsMsgChannel_Sync	61.62 us	0.624 us	0.553 us	-	-
RunInFlightNatsMsgChannel_Sync	57.67 us	0.394 us	0.368 us	-	-
RunNatsMsgChannel_Sync_Pulse	60.45 us	0.202 us	0.158 us	-	-
RunInFlightNatsMsgChannel_Sync_Pulse	56.52 us	0.275 us	0.258 us	-	-
RunNatsMsgChannel_Async_Pulse	88.67 us	0.274 us	0.229 us	-	-
RunInFlightNatsMsgChannel_Async_Pulse	94.57 us	0.498 us	0.441 us	-	-
RunNatsMsgChannel_Async_Pulse_Unhappy	192.28 us	1.355 us	1.201 us	31.2500	147456 B
RunInFlightNatsMsgChannel_Async_Pulse_Unhappy	245.43 us	4.011 us	3.752 us	27.8320	131072 B
RunNatsMsgChannel_Async_Unhappy	123.47 us	1.597 us	1.494 us	-	900 B
RunInFlightNatsMsgChannel_Async_Unhappy	128.66 us	1.880 us	1.666 us	-	833 B
RunNatsMsgChannel_Async_Happy	117.61 us	0.932 us	0.872 us	-	408 B
RunInFlightNatsMsgChannel_Async_Happy	121.40 us	0.817 us	0.764 us	-	408 B