fix: POCOObservableForProperty having a race condition

[worldbeater]

What kind of change does this PR introduce?

Fixes the POCOObservableForProperty.GetNotificationForProperty concurrent access error discovered at reactiveui/ReactiveUI.Validation#21

What is the current behavior?

When running unit tests in parallel in ReactiveUI.Validation.Tests project, some of the tests fail randomly with the InvalidOperationException. The same thing happened on Azure Pipelines CI, but only on Mac machine with 1 build of 4 total.

System.InvalidOperationException : Operations that change non-concurrent collections must have exclusive access. A concurrent update was performed on this collection and corrupted its state. The collection's state is no longer correct.
   at System.Collections.Generic.Dictionary`2.TryInsert(TKey key, TValue value, InsertionBehavior behavior)
   at ReactiveUI.POCOObservableForProperty.GetNotificationForProperty(Object sender, Expression expression, String propertyName, Boolean beforeChanged) in D:\a\1\s\src\ReactiveUI\ObservableForProperty\POCOObservableForProperty.cs:line 39
   at ReactiveUI.ReactiveNotifyPropertyChangedMixin.NotifyForProperty(Object sender, Expression expression, Boolean beforeChange) in D:\a\1\s\src\ReactiveUI\Mixins\ReactiveNotifyPropertyChangedMixin.cs:line 206
   at ReactiveUI.ReactiveNotifyPropertyChangedMixin.NestedObservedChanges(Expression expression, IObservedChange`2 sourceChange, Boolean beforeChange) in D:\a\1\s\src\ReactiveUI\Mixins\ReactiveNotifyPropertyChangedMixin.cs:line 190
   at System.Reactive.Linq.ObservableImpl.Select`2.Selector._.OnNext(TSource value) in D:\a\1\s\Rx.NET\Source\src\System.Reactive\Linq\Observable\Select.cs:line 39

at System.Collections.Generic.Dictionary`2.FindEntry(TKey key)
   at ReactiveUI.POCOObservableForProperty.GetNotificationForProperty(Object sender, Expression expression, String propertyName, Boolean beforeChanged) in D:\a\1\s\src\ReactiveUI\ObservableForProperty\POCOObservableForProperty.cs:line 33
   at ReactiveUI.ReactiveNotifyPropertyChangedMixin.NestedObservedChanges(Expression expression, IObservedChange`2 sourceChange, Boolean beforeChange) in D:\a\1\s\src\ReactiveUI\Mixins\ReactiveNotifyPropertyChangedMixin.cs:line 190
   at System.Reactive.Linq.ObservableImpl.Select`2.Selector._.OnNext(TSource value) in D:\a\1\s\Rx.NET\Source\src\System.Reactive\Linq\Observable\Select.cs:line 39

The POCOObservableForProperty internal static Dictionary seems to be the cause.

ReactiveUI/src/ReactiveUI/ObservableForProperty/POCOObservableForProperty.cs

Lines 19 to 21 in eb53eee

	public class POCOObservableForProperty : ICreatesObservableForProperty
	{
	private static readonly Dictionary<(Type, string), bool> hasWarned = new Dictionary<(Type, string), bool>();

What is the new behavior?

Now, the Dictionary is replaced with ConcurrentDictionary, which can handle the race condition.

What might this PR break?

Nothing.

[codecov]

Codecov Report

Merging #2020 into master will increase coverage by 0.04%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2020      +/-   ##
==========================================
+ Coverage   60.74%   60.78%   +0.04%     
==========================================
  Files         115      115              
  Lines        4580     4580              
  Branches      656      656              
==========================================
+ Hits         2782     2784       +2     
+ Misses       1617     1614       -3     
- Partials      181      182       +1

Impacted Files	Coverage Δ
...ObservableForProperty/POCOObservableForProperty.cs	100% <100%> (ø)	⬆️
src/ReactiveUI.Winforms/RoutedViewHost.cs	90.27% <0%> (+2.77%)	⬆️

---

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 a941bda...3e7fe80. Read the comment docs.