Serialization / deserialization is not thread safe

[alxmitch]

Describe the bug
Concurrently deserializing or serializing the same type from multiple threads can fail. Example stack trace:

Operations that change non-concurrent collections must have exclusive access. A concurrent update was performed on this collection and corrupted its state. The collection's state is no longer correct.

   at System.ThrowHelper.ThrowInvalidOperationException_ConcurrentOperationsNotSupported()
   at System.Collections.Generic.Dictionary`2.TryInsert(TKey key, TValue value, InsertionBehavior behavior)
   at System.Collections.Generic.Dictionary`2.set_Item(TKey key, TValue value)
   at YamlDotNet.Serialization.ObjectFactories.DefaultObjectFactory.GetStateMethods(Type attributeType, Type valueType)
   at YamlDotNet.Serialization.ObjectFactories.DefaultObjectFactory.ExecuteState(Type attributeType, Object value)
   at YamlDotNet.Serialization.ObjectFactories.DefaultObjectFactory.ExecuteOnDeserializing(Object value)
   at YamlDotNet.Serialization.NodeDeserializers.ObjectNodeDeserializer.Deserialize(IParser parser, Type expectedType, Func`3 nestedObjectDeserializer, Object& value)
   at YamlDotNet.Serialization.ValueDeserializers.NodeValueDeserializer.DeserializeValue(IParser parser, Type expectedType, SerializerState state, IValueDeserializer nestedObjectDeserializer)

To Reproduce
Serialize / deserialize objects of the same type from multiple threads using the same Serializer / Deserializer
See kubernetes-client/csharp#1537

[alxmitch]

#920

[filzrev]

Add related discussion link (#844)

Currently I'm using the ThreadLocal<ISerializer> workaround though.
I'd appreciate if a thread-safe implementation is also supported.