SessionDynamicStore is not thread-safe; in particular,
if 2 threads try to MoveToMemory at the same time,
problems happen. Thread locks are needed.
Also, it unnecessarily accesses the filesystem on
every request, even if the session is in memory.
Most users won't notice these flaws, but one user
reported problems with a frameset where multiple
requests for the same session came in at precisely
the same time.
Some code cleanup would also be nice.
(Geoff is currently working on fixing this.)