Use sync.RWMutex in sessionData to allow concurrent readers

[pete-woods]

• I have a custom store that wants to read domain-specific values out of the session (to maintain a table associating users with sessions, so they can be listed and deleted efficiently).
• At present stores cannot access the session data, as this causes a deadlock on the sync.Mutex.
• I've tried to carefully go through each accessor of the mutex and convert those that perform only read operations to RWLocks.
• I'd expect this is also a small general performance improvement in any situation where you're reading from the session data concurrently.