Fix #498

[SilverFox (yyjdelete)]

More details can be see in #498

In fact, AttributeKey can also be public and move all static method into the new static class(In fact AttributeKey<T>.Exists(string) never do check for typeof(T)), like Tuple.Create<T1, T2>() and Tuple<T1, T2> does. But it will be an breaking change.

[SilverFox (yyjdelete)]

Some test failed with the below message. Seems just an concurrent issue with InternalLoggerFactoryTest, and not related to this PR, will push again to check this. Not sure why DisableParallelization = true not works.
System.TypeInitializationException : The type initializer for 'DotNetty.Common.Utilities.HashedWheelTimer' threw an exception.

Details

---- Moq.MockException : ILoggerProvider.CreateLogger("DotNetty.Common.Utilities.HashedWheelTimer") invocation failed with mock behavior Strict.
All invocations on the mock must have a corresponding setup.
Stack Trace:
   at DotNetty.Common.Utilities.HashedWheelTimer..ctor(TimeSpan tickInterval, Int32 ticksPerWheel, Int64 maxPendingTimeouts) in C:\projects\dotnetty\src\DotNetty.Common\Utilities\HashedWheelTimer.cs:line 105
   at DotNetty.Common.Tests.Utilities.HashedWheelTimerTest.TestTimerShouldThrowExceptionAfterShutdownForNewTimeouts() in C:\projects\dotnetty\test\DotNetty.Common.Tests\Utilities\HashedWheelTimerTest.cs:line 86
----- Inner Stack Trace -----
   at Moq.FindAndExecuteMatchingSetup.Handle(Invocation invocation, Mock mock) in C:\projects\moq4\src\Moq\Interception\InterceptionAspects.cs:line 145
   at Moq.Mock.Moq.IInterceptor.Intercept(Invocation invocation) in C:\projects\moq4\src\Moq\Interception\Mock.cs:line 21
   at Castle.DynamicProxy.AbstractInvocation.Proceed()
   at Castle.Proxies.ILoggerProviderProxy.CreateLogger(String categoryName)
   at Microsoft.Extensions.Logging.Logger..ctor(LoggerFactory loggerFactory, String name)
   at Microsoft.Extensions.Logging.LoggerFactory.CreateLogger(String categoryName)
   at DotNetty.Common.Internal.Logging.InternalLoggerFactory.GetInstance(String name) in C:\projects\dotnetty\src\DotNetty.Common\Internal\Logging\InternalLoggerFactory.cs:line 94
   at DotNetty.Common.Utilities.HashedWheelTimer..cctor() in C:\projects\dotnetty\src\DotNetty.Common\Utilities\HashedWheelTimer.cs:line 21