net: conn: Improve thread safety in connection module

Iterating over connection list w/o mutex lock could lead to a crash on
constant incoming packet flow. Fix this by:

1. Adding mutex lock when iterating over an active connection list, to
   prevent list corruption.
2. Create a copy of the callback and user data pointers before releasing
   lock, to prevent NULL pointer dereference in case connection is
   released before callback is executed.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>