Windows: since Fleck is thread-safe, we may not need _socketLock

[cwillisf]

Expected Behavior

We should only use thread synchronization when necessary.

Actual Behavior

In Session.cs, interaction with _webSocket is protected by locking a semaphore called _socketLock. Fleck, the WebSocket library we use, claims to be thread-safe so we may not need this semaphore. Removing it may improve performance slightly. See also statianzo/Fleck#191.

Thanks to @terado0618 for bringing this up!