Make C++ Session object thread safe

[hvge]

We should make methods in C++ class Sessionthread safe. We had a several discussions about this topic and it looks like that the best solution is to add the synchronization directly to the low level Session object. The original idea was about to synchronize just the signature calculation operations (due to signature counter increment). This seems to be insufficient, because the lifetime of the Session object is typically very long and we're reusing the object for the whole lifetime of PowerAuthSDK object. The long lifetime means higher chance to get an obscure race condition.

Looks like that the sufficient solution is to add a guard-locks to the public Session's methods. So, the change will affect all supported platforms.

Important for IOS

The PA2Session stores the last error code to the object instance. That's also wrong and the interface should be changed. We have a related ticket #49 for that.