OSD: fill in Session in ms_handle_connect

Signed-off-by: Samuel Just <sam.just@inktank.com>
OSD: set up Session in ms_handle_accept

Signed-off-by: Samuel Just <sam.just@inktank.com>
RWLock: assert pthread function return values

Signed-off-by: Samuel Just <sam.just@inktank.com>