The "exit" actions were not being saved with the correct session
token, because on.exit() calls inside of withReactiveDomain() get
executed *after* the end of the withReactiveDomain.
Observers were being prevented from being garbage collected by
their own onReactiveDomainEnded() event handlers. This commit
fixes that by making sure that those event handlers are only
registered when autoDestroy=TRUE, and that they are unregistered
both on destruction and when autoDestroy is changed.
