#57 Context cache on server never deletes contexts

[Marc Novakowski]

The class KeelAbstractServer maintains a cache of
Context objects in a private Map called "contexts".
Various methods in KeelAbstractServer add or look up
contexts in the cache, but none actually remove
contexts. This means that as more and more contexts
are added, the server could run out of memory
(especially if the contexts contain a large number of
items).

As suggested by Michael Nash, it would probably be a
good idea to have some sort of context-removal
algorithm (i.e. configurable timeouts) to prevent this
from happening.