Periodically purge disposals

[mxgrey]

For very long-running tasks it's possible that disposal information may pile up arbitrarily high without being helpful. This could especially be a problem if a workflow is used as the main loop of an application.

We should provide a way for disposals to be periodically purged even while a workflow session is still active, especially for disposals that are very old or whose nodes have been revisited.

Perhaps we could choose to only retain the latest disposal of a node per session, or put a cap on how many disposals are stored per node per session.