Convert ltt_ust_session to c++

Convert ltt_ust_session to c++

This is a minimal patch to move the ltt_ust_session struct to c++ style.
This will allow us to add complex type in the struct later on (such
as unique_ptr/shared_ptr etc).

Use default member initialization when possible.

Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Change-Id: I2578fa24fde8de0728d20a938aa55e2a6ad7a161