add ZKMod project

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This is a very early idea that may not eventually be realized. To resolve early design mistakes in zkweb project, I want to create a new project called ZKWebZero, it should contain the following features:

• Supports automatic resolution of dependencies between plugins
• Supports automatic discover plugins in specified directories
• Support real hot reload of plugins (without restarting process)
  • This can be very tricky, for example:
  • A provides IService, B provides ServiceImpl, C uses IService, then
  • When A changed, all of A, B, C should recompile and reload
  • When B changed, service entries in the container should be replaced, and proxy instances of IService should invalidate the real target and resolve it again from the container, we can track proxy instances by weak references
  • basically we need:
  • module aware container
  • proxy class of service types (require interface or all public members are virtual)
  • support custom logic on plugin load and unload
  • and yeah, memory leak will happen because .net doesn't really support assembly unload, we still need restart the process under certain conditions
• Separate functions to plugins such as template, orm and mvc, the core should only contains plugin system and http request entry point
• No global veriables at all, that means we need to eliminate the class Application
• Use xunit for unit testing
• Provide legacy layer in Framework.ZKWebLegacy plugin for compatibility with legacy plugins
• Provide asp.net core and razor template engine support in Framework.AspnetCore plugin
• Provide nancy support in Framework.Nancy plugin

addition ideas

• mvcc reference counted container maybe a better solution
• rename the term plugin to module
• test app domain unloading on netcore 2.0 with linux, if pass then test object serializing
• if plugins need to be managed in single app domain, decide how to load multi version assemblies
• rewrite old plugins of legacy zkweb is unacceptable

This is a long term job, you can add your opinion if you saw this issue, but don't expect it to be available soon.

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AppDomain.CreateDomain is unavailable on linux.

Unhandled Exception: System.PlatformNotSupportedException: Secondary AppDomains are not supported on this platform.
   at System.AppDomain.CreateDomain(String friendlyName)
   at appdomaintest.Program.Main(String[] args) in /home/ubuntu/Projects/appdomaintest/Program.cs:line 11

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wait for these issues, maybe available on .net core 2.2

• github.com/dotnet/coreclr/issues/16925
• github.com/dotnet/coreclr/issues/552

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Changed idea, the project name should be ZKMod and it should only contains the module system. After refractory there will be 3 primary project: ZKWeb, ZKStandard, ZKMod, and ZKMod should be independent. The final decision is load all plugins in same appdomain or loadcontext, and unload them all when reload any module. I don't have much time now, I would like to start working on it after november.

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Final solution: no ZKMod project, .NET Core 3.0 collectible assembly is used in ZKWeb 3.0 and there no hot reloading support for .NET Framework because microsoft set it dead.