vamsas sync fails if user deletes an alignment shared from another vamsas app that then updates it

Jalview doesn't track alignments deleted from the workspace that originated from a shared vamsas document.

Steps to reproduce:
1. Start two jalviews. connect them to a new vamsas session
2. import an alignment. copy and paste alignment to new window. share to session.
3. close one alignment in the jalview that imported changes from the session.
4. select 'session update' in the original jalview again
-> alignment will be shared to other jalview again, but 'updating vamsas session' progress bar will not disappear

Stacktrace here:

ERROR - Serious! Client Error during handling of uk.ac.vamsas.client.events.documentUpdateEvent
java.lang.Error: IMPLEMENTATION ERROR: old jalview object is not bound ! (jalview.bin.Jalview_3914731050.157)
at jalview.io.VamsasAppDatastore.replaceJvObjMapping(VamsasAppDatastore.java:1470)
at jalview.io.VamsasAppDatastore.flushAlignViewports(VamsasAppDatastore.java:1429)
at jalview.io.VamsasAppDatastore.updateJalviewFromAppdata(VamsasAppDatastore.java:1366)
at jalview.gui.VamsasApplication.updateJalview(VamsasApplication.java:395)
at jalview.gui.VamsasApplication.dealWithDocumentUpdate(VamsasApplication.java:594)
at jalview.gui.VamsasApplication$2.propertyChange(VamsasApplication.java:634)
at java.beans.PropertyChangeSupport.firePropertyChange(PropertyChangeSupport.java:339)
at java.beans.PropertyChangeSupport.firePropertyChange(PropertyChangeSupport.java:276)
at uk.ac.vamsas.client.simpleclient.EventGeneratorThread._raise(EventGeneratorThread.java:136)
at uk.ac.vamsas.client.simpleclient.EventGeneratorThread.documentChanged(EventGeneratorThread.java:182)
at uk.ac.vamsas.client.simpleclient.EventGeneratorThread$1.handleWatchEvent(EventGeneratorThread.java:97)
at uk.ac.vamsas.client.simpleclient.WatcherElement.callHandler(WatcherElement.java:131)
at uk.ac.vamsas.client.simpleclient.WatcherElement.doWatch(WatcherElement.java:118)
at uk.ac.vamsas.client.simpleclient.VamsasFileWatcherThread.run(VamsasFileWatcherThread.java:114)
INFO - Recovering from errors or exceptions generated by client application