Memory leaks in yjs websocket docs map

[sbavoma]

I did some profiling on our yjs websocket server and found that there is a memory leak present in there.
Even after the yjs ws session has ended, the library fails to de-reference its Yjs Doc objects leading them to pile up in memory.

After careful debugging I have found that here is the issue - https://github.com/yjs/y-websocket/blob/master/bin/utils.js#L66
Perhaps we should use a weakmap? Is there any way to make sure when we call delete on this map we destroy all references.

[jiangxiaoqiang]

I am facing the same issue.