Make the session inside a stream dispose-safe
Make the session inside a stream dispose-safe

- Protect it with its own mutex
- Take a ref when using it
- Unref afterwards
- Remove now useless disposed variable

tieto/farstream,ahmedammar/skype_farsight2,tieto/farstream,kakaroto/farstream,tieto/farstream,ahmedammar/skype_farsight2,kakaroto/farstream,tieto/farstream,kakaroto/farstream,pexip/farstream,ahmedammar/skype_farsight2,pexip/farstream,shadeslayer/farstream,pexip/farstream,ahmedammar/skype_farsight2,pexip/farstream,shadeslayer/farstream,tieto/farstream,shadeslayer/farstream,kakaroto/farstream,shadeslayer/farstream