Merge Tree: Rationalize registers about fluidframework HOT 7 CLOSED

I see their benefit, we don't need to pass data over the wire if clients already have it, but we'd need to fix all the problems, and i'm not sure it is worth it at the moment.

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Copy also never worked right, as it will mess up the pending segment group queue, as a segment group is never added for the local copy op, so when it op is acked it will ack the wrong pending segment group

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Another thought i had is this has the potential for denial of service attacks. Imagine a large document, like pride and predudice. I could open the document, copy the entire thing, and then close my client. Then repeat. This would result is the entirety of the document being stored in a register for each client i do this with, which would eventually exhaust the memory on the connected clients. This scenerio assumes a malicious user, but something similar could take place even with a non-malicious client.

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Registers avoid large ops for cut copy paste. The sequence groups are going away soon to be replaced by general continuous ops.

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I agree we need to finish the feature including cleanup semantics, but we definitely need the feature to avoid arbitrary-sized ops for things like copying the whole document and pasting it.

We could tie registers to the lifetime of quorum members so that the register would be deleted when its creator leaves the quorum and the op that created the register is <= min seq.

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        if (range.end) {
            this.copy(range, op.register, clientArgs);
            // enqueue an empty segment group to be dequeued on ack
            //
            if (clientArgs.sequenceNumber === UnassignedSequenceNumber) {
                this.mergeTree.pendingSegments.enqueue({segments: []});
            }
            return true;
        }

This code from applyInsertOp seems to be setting up the empty segment group to be acked in the local copy case.

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This issue is 2 years old. Closing. @anthony-murphy - if we need some cleanup due to not supporting registers right now, please open separate issue,

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