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.
• I've had this in production for some time now with no issues.

[alexedwards]

@pete-woods I'm not sure that we an use RLock in the Commit function --- sd is written to in this line:

scs/data.go

Line 101 in 9b07998

if sd.token, err = generateToken(); err != nil {

.

[pete-woods]

Thanks - will have a look. We've been relying on a version of this in production for a while that lets a custom session store read the session info from the context.