Starting from 3f83469c Conn and WatchLink started to inherit from
interface, which made them to use virtual functions, which, without
destructor being also virtual emits the following warnings:

    wcfs/client/wcfs.cpp: In member function ‘virtual void wcfs::_Conn::decref()’:
    wcfs/client/wcfs.cpp:1531:16: warning: deleting object of polymorphic class type ‘wcfs::_Conn’ which has non-virtual destructor might cause undefined behavior [-Wdelete-non-virtual-dtor]
             delete this;
                    ^~~~

    wcfs/client/wcfs_watchlink.cpp: In member function ‘virtual void wcfs::_WatchLink::decref()’:
    wcfs/client/wcfs_watchlink.cpp:514:16: warning: deleting object of polymorphic class type ‘wcfs::_WatchLink’ which has non-virtual destructor might cause undefined behavior [-Wdelete-non-virtual-dtor]
             delete this;
                    ^~~~