MODE-1830 Corrected the purging of workspace caches in a cluster

Before this fix, when a change was made via a WritableSessionCache,
any changes persisted to the store were also used to directly purge
any previously-cached node representations from the corresponding
WorkspaceCache. Thus, when the RepositoryCache received the changes,
it simply forwarding them to all workspaces except the one in which
the changes originated.

However, in a clustered environment, the changes originate on one
process and are sent to all other processes. The logic described
above works fine in the same process in which the changes originated,
but it doesn't correctly propagate the changes in the other processes.

The fix was pretty simple: determine if the changes came from the same
process. If so, the use the current logic. If not, always forward the
changes to all WorkspaceCache instances.

I also modified the ClusteredRepositoryTest to look in process A for
nodes changed in process B. This failed before the logic was fixed,
and now works with the fix.