epee: adaptive connection timeout system

a connection's timeout is halved for every extra connection
from the same host.

Also keep track of when we don't need to use a connection
anymore, so we can close it and free the resource for another
connection.

Also use the longer timeout for non routable local addresses.

contrib/epee/include/net/net_utils_base.h

@@ -281,6 +281,7 @@ namespace net_utils
 	{
 		virtual bool do_send(const void* ptr, size_t cb)=0;
     virtual bool close()=0;
+    virtual bool send_done()=0;
     virtual bool call_run_once_service_io()=0;
     virtual bool request_callback()=0;
     virtual boost::asio::io_service& get_io_service()=0;