#58 Topic deletion takes a long time

[Kal Ahmed]

Topic.destroy takes a long time to complete - this is
especially noticeable when deleting many topics at the
same time. Reported timing is 300ms on 700MHz machine
for destroying one topic.

This is most probably related to the consistency
checking code which ensures that a topic is not being
used as a type, roleSpec or a theme in a scope before
allowing the deletion.

In the in-memory and Ozone backends, the solution could
be to add a reference count or reference list to the
Topic object or to maintain a reference count table on
the parent TopicMap object (might be easier to do,
especially in Ozone where event notifications can get
split between client and server sides).