Thread-safe Session

[ecorm]

Consider making Session objects more thread safe. This might entail that internal::WampMessage objects can no longer be reused in internal::Client, but must be constructed every time a message needs to be sent.

[ecorm]

Now that serializer/deserializer instances are reused within the same Session, making its existing operations reentrant is impossible without posting everything to Session's strand.

A set thread-safe methods (or overloads) will be added to Session which post everything to Session's strand. That way, single-threaded apps can continue using the existing API without the post penalty.

[ecorm]

Completed in 29153f6

Session now has overloads taking the ThreadSafe tag which will perform the operation in a thread-safe manner when invoked from concurrent threads,