Register Middlewares as Transient to avoid clobber

[gkinsman]

I noticed this issue due to a failing test in JustSayingStack.

Most of our tests only establish a single subscription, which works fine. If there are multiple subscriptions however, then the second will resolve the same instance of LoggingMiddleware or SqsPostProcessorMiddleware (the only middlewares that are resolved from DI). As those instances were already used to build the previous pipeline, they are already part of an existing pipeline, and therefore have _next values that link them to the next step. When the second builder comes along and resolves the same middleware, it redirects the _next to the second subscription. The end result is that the same handler is invoked for all subscriptions. Bad!

This PR does the following to fix this:

• Adds a test to ensure this doesn't happen
• Changes the middleware registrations to be transient
• Emits an InvalidOperationException when constructing middlewares if the _next property has already been populated, which indicates that the middleware has been incorrectly registered and the single instance is already part of an existing pipeline. If there's only one subscription this won't throw, so it still works for apps that only have a single sub and single instance middlewares.

[codecov]

Codecov Report

Merging #958 (48bd39b) into main (1ed1c34) will decrease coverage by 0.09%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##             main     #958      +/-   ##
==========================================
- Coverage   84.37%   84.27%   -0.10%     
==========================================
  Files         131      131              
  Lines        3199     3206       +7     
==========================================
+ Hits         2699     2702       +3     
- Misses        500      504       +4     

Flag	Coverage Δ
linux	84.27% <66.66%> (-0.10%)	⬇️
macos	53.09% <40.00%> (-0.03%)	⬇️
windows	53.09% <40.00%> (+0.26%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...ging/Middleware/Handle/HandlerMiddlewareBuilder.cs	83.78% <42.85%> (-9.77%)	⬇️
...njection.Microsoft/IServiceCollectionExtensions.cs	86.73% <100.00%> (ø)
...ndencyInjection.StructureMap/JustSayingRegistry.cs	100.00% <100.00%> (ø)
...ustSaying/Messaging/Middleware/MiddlewareBase`2.cs	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 1ed1c34...48bd39b. Read the comment docs.