ost::Socket: add compatibility with ucommon::Socket::address

As a proposed transition mechanism for https://github.com/dyfet/ccrtp/issues/11, this commit makes ost::Socket closer to ucommon::Socket by supporting ucommon::Socket::address for getSender(), getLocal(), getPeer(), join() and drop().

The change is sightly breaking the commoncpp API: calling  socket::getSender(), getLocal() or getPeer() without argument now returns a ucommon::Socket::address instead of IPV4Host.

Those methods where already moved to getIPV4Sender() etc in the past, and calling the methods with a port argument will still work as expected, so the change appeared reasonable to me, as it would help to reduce the volume of change required in CCRTP and commoncpp to switch to ucommon.

The API change does not break CCRTP.