Fix: liblttng-ust process startup hang when sessiond is stopped

Ensure the listener thread owns socket and notify_socket, so they don't
have to hold the ust_lock() while connecting to the sessiond and reading
from this socket.

Therefore, after process fork, we can safely cleanup those retources,
because the thread has been removed by the operating system. On exit,
however, let the OS teardown those sockets, so exit path does not race
with the listener thread.

Fixes #545

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>