[soft-debugger-session] Domain unload type fix.

[lukaszunity]

Maintain list of loaded assemblies for each domain to remove unloaded types correctly on domain unload.

As discussed in issue #57

[lukaszunity]

I've updated this PR also remove types on Assembly Unload, to match the logic before opening this PR.

[DavidKarlas]

Why is removal when Assembly Unload event happens needed? Won't UnloadDomain happen soon after?

Only thing that annoys me atm with this PR... is that when 10 assemblies are loaded at start(one after another) of program we will be executing extra SDB commands to fetch assemblies in domains 10 times... Maybe we could lazy fetch domain assemblies 1st time some type is loaded(if code gets too ugly... maybe we can live with existing solution in this PR)

I also noticed that types that need unloading are determined by assembly location...(this is existing code)
Doesn't this mean that if type System.Net.Socket was loaded in DomainRoot and is part of /path/to/System.dll will be unloaded when some other AppDomain is unloaded, which shouldn't be?

[lukaszunity]

You are right that types might be incorrectly unloaded in the wrong domain and in fact, if the some other AppDomain contains all the assemblies that are in DomainRoot, then this PR is equivalent to doing types.Clear() HandleAppDomainUnloadEvents when unloading the other domain.

I don't think there is an good way to solve this issue domain unload issue without updating the debugger agent to send the domain ids for type loads.

I will fix this enum evalution issue in our own fork with a work-around and then revisit the issue with a the proper fix in the future.