fix(session tracker): fix synchronization of de-duplication code

[tomlongridge]

Goal

Fixes #750 - makes the list checking logic in session sending thread safe.

Changeset

• Delivery/BugsnagSessionTrackingApiClient.m - used NSSet's containsObject rather than iterate ourselves and wrapped the list mutation in a synchronized block.
• Delivery/BugsnagSessionTrackingApiClient.m - also updated the log message to show session ID rather than a count (as this is always 1 now)

Tests

Load-tested calling startSession across multiple threads:

    for (int i = 1; i <= 200; i++)
    {
        dispatch_queue_t _processingQueue = dispatch_queue_create("CrashReporting", DISPATCH_QUEUE_SERIAL);
        dispatch_async(_processingQueue, ^{
            [Bugsnag startSession];
        });
    }