some cleanup and better metadata support

src/csharp/Grpc.Core/Internal/AsyncCall.cs

src/csharp/Grpc.Core/Internal/BatchContextSafeHandle.cs

src/csharp/Grpc.Core/Internal/MetadataArraySafeHandle.cs

src/csharp/Grpc.Core/Server.cs

src/csharp/ext/grpc_csharp_ext.c