Replace the Dictionary with HashSet in AppDomainShutdownMonitor

[lindexi]

Benchmark:

Method	argList	Mean	Error	StdDev	Ratio	RatioSD
'AddAndRemove Old'	Syste(...)ence] [55]	679.7 us	10.42 us	8.70 us	1.00	0.00
'AddAndRemove New'	Syst(...)er]] [113]	667.7 us	11.07 us	10.35 us	?	?

Method	argList	Mean	Error	StdDev	Ratio	RatioSD
'AddBeforeRemove Old'	Syste(...)ence] [55]	1.414 ms	0.0174 ms	0.0154 ms	1.00	0.00
'AddBeforeRemove New'	Syst(...)er]] [113]	1.197 ms	0.0097 ms	0.0081 ms	?	?

Method	Mean	Error	StdDev	Median	Ratio	RatioSD
'AddOnly Old'	2.886 ms	0.0574 ms	0.1160 ms	2.821 ms	1.00	0.00
'AddOnly New'	2.880 ms	0.0639 ms	0.1865 ms	2.827 ms	1.00	0.09

Benchmark Code: https://github.com/lindexi/lindexi_gd/tree/b63ba44c2077bcc4136455f16b309fe74a288dcc/AppDomainShutdownMonitor

[ryalanms]

Thanks again, @lindexi.