Lock reads and writes to session map

[morganchen12]

Potentially fix #1964.

TAP runs over at cr/217930814.

[paulb777]

Any particular reason for NSLock versus GCD or @synchronized?

[morganchen12]

I wanted an explicit object to lock/unlock on rather than an implicit lock associated with a class instance or something. This isn't a hugely important decision, and I can rewrite to use @synchronized if preferable.

[ryanwilson]

I think this may be causing a deadlock based on Travis. Please try to catch this locally if you can to make sure we aren't causing a deadlock anywhere.

Using NSLock is good with me, or I started an implementation with GCD if locking is causing issues (I'd prefer to avoid @synchronized).

[morganchen12]

Fixed deadlock and passed travis.