Tidy up the general proxy logic by moving common functionality
into libcoap.
New functions coap_proxy_forward_request(), coap_proxy_forward_response()
and coap_proxy_is_supported() added.
New --enable-proxy-code support added to configure.
New ENABLE_PROXY_CODE support added to CMakeLists.txt.
Support for common shared connection to next hop.
Support for round-robin balancing across multiple next hops.
