Force IAPSession assert on main queue

[joeljfischer]

Bug Report

The IAPSession `stop code currently contains the following line:

NSAssert([NSThread isMainThread], @"stop must be called on the main thread");

However, due to the RPC response queue changes in #813 we are seeing rare cases where stop will be called off the main thread. We should wrap it to force it onto the main queue if it is not via dispatch_sync to ensure timing remains the same.

Reproduction Steps

Unknown

Expected Behavior

The stop succeeds

Observed Behavior

The stop fails

OS & Version Information

• iOS Version: n/a
• SDL iOS Version: 5.1.0 develop
• Testing Against: TDK