5.0.0 fix memory leak

[mccraigmccraig]

yapster APIs and streaming processes were leaking - OOMEing over a couple of days (of low traffic on internal dev instances)

investigation showed lots of our EntityInstance db records referenced from the global downstream graph metadata in manifold.stream.graph/handle->downstreams

this was something of a mysterious issue, in that usually stream-related leaks are because some streams are not being closed, and things stop working because streams are not closed and stream/reduces don't terminate - but in this case streams were apparently being closed and correct operation of the system was observed

two issues were found which are fixed in this PR

1. qbits.alia.manifold/handle-page-completion-stage recurses to fetch the next page. since manifold often uses the caller thread for callbacks, this could blow the stack, so it has been changed to use deferred/loop
2. qbits.alia.manifold/execute-stream was using a (stream/transform (mapcat identity) <page-stream>) to concatenate pages from the page-stream on to a record-stream. this appears to be responsible for the leak (replacement of the stream/transform has fixed the leak)