8886: Experimental asynchronous permissions update.

8886: Experimental asynchronous permissions update.

Add configuration parameter 'async_permissions_update' (default false).  If
true, do not delete permission cache in #invalidate_permissions_cache, but
instead trigger "NOTIFY invalidate_permissions_cache" on the database.

Add script/permission-updater.rb which runs as an independent process.  It
blocks on "LISTEN invalidate_permissions_cache" and updates the permission
cache whenever notified.

This is not ready for use; in particular it creates a race condition
recomputing permissions with effects such as not being able to read back API
records that were just created.