Addressing synchronization issues in InMemoryStorage class with the f…

[mike-kaufman]

Addressing synchronization issues reported in Issue #82 in InMemoryStorage class with the following changes:

• added ReaderWriterLockSlim to control access to InMemoryStorage's dictionary & list that track individual RequestInfo instances
• added ReaderWriterLockSlim to control access to each individual RequestAccess
• removed RequestInfo's "Messages" accessor, and replaced it with a SnapshotMessages() method that will make a copy of the current set of messages. This is to avoid "collection has been modified" exceptions where the underlying set of messages is modified while a previous call is iterating over the list.
• Made RequestIndices instances immutable.
• added a bunch of asserts to validate assumptions about locking of resources.
• added a test that was triggering the reported "collection modified" exceptions.

Please let me know if there's questions or feedback.

[mike-kaufman]

@avanderhoorn, @nikmd23 - this is waiting on your review/feedback. Let me know if you have questions/comments/suggestions.