Improve GraphQL subscriptions handling

[arboleya]

It has been observed that sometimes subscriptions are left open.

We must ensure all subscriptions are closed to avoid bloating the nodes.

Quoting @Voxelot:

• My assumption was that the node creates a buffer for each subscription that hangs around either until 1) it is fuly consumed or 2) a timeout occurs and the buffer is dropped.
  The subscription wouldn't automatically close immediately when the final state comes, because if it didn't wait for the sdk to consume the messages then the buffer would close before the user sees the messages. So if we have a lot of junk subscriptions hanging around, the problem is either with 1 or 2. Ie. there's something in the frontend opening subscriptions but ignoring the result and not consuming the messages or the client isn't properly closing the buffer when all the messages are read.

Follow-up after:

• Provide a way to submit TXs without using subscriptions #3683