Get the context with the given id, optionally marking the context as
being in use.
param
key the context's id
param
markInUse if true, marks any returned context as
being in use. If false, will return
contexts that are marked as removed; otherwise will
throw NoSuchEJBException if such a context is found
return
the context
throws
NoSuchEJBException if no context with the given id exists or
if the context exists but has been marked
as removed and markInUse is
true