Support multiplexing with graphql-ws subprotocol

[MohamedSabthar]

Description:
Multiplexing is not supported by the current graphql subscription implementation when utilizing the graphql-ws/graphql-transport-ws subprotocol.

To handle the multiplexing properly, the incoming messages to the onTextMessage handler must be processed concurrently.
Regarding the current behaviour of the onTextMessage handler, this handler is not dispatched concurrently for received text messages, rather it gets dispatched frame by frame while waiting for the completion of the preceding onTextMessage call. To handle the text message concurrently, we may add a worker inside the onTextMessage handler.

By making this change, the handler will be able to get the complete message even when messages are in-flight to the client.
Once the server has received this complete message from the client, it can stop processing the stream. If we don't stop processing the stream after receiving the complete message, the server will max out with the load. (Current implementation doesn't handle this scenario)

Note: Adding a worker inside the onTextMessage function would necessitate converting every Node class in the graphql package into an isolated class, which might result in performance degradation.

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[ThisaruGuruge]

We might be able to do this by making all the parser nodes as readonly classes. Since there is another requirement to do so, I think we should go for it. This needs some changes in the parser as well. Let's break down this task. First, let's try to make the parser nodes readonly by creating them once we have all the information. Then we can proceed with the next step, which is to use a worker inside the onTextMessage() method.

[MohamedSabthar]

Sub tasks

• Make all document tree nodes readonly
• Update the parser implementation to generate an immutable tree
• Refactor the visitor implementations which mutates the nodes of the tree.
  - Visitors should create a new readonly node and store them on a map (which holds the modified nodes)
  - When visiting the node visitors should visit the modified node ( if a node modified by a previous visitor)
• Re-build the document tree with the modified nodes
• Implement the multiplexing support using worker
• Load test