Deal with "infinite list" held by memorystream.cursor

[jmalloc]

If you open a memory stream cursor and never close it, you are essentially holding the original head of the linked-list buffer, so you will keep those objects alive forever.

For the most case this doesn't really matter as the engine is in charge of its own consumers, but this is not true for the networkserver.

Ultimately we need some kind of timeout between calls to next, but I'm not sure how best to implement this yet.