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.

tests/fuzz/levin.cpp

@@ -158,6 +158,7 @@ namespace
     }
 
     virtual bool close()                              { return true; }
+    virtual bool send_done()                          { return true; }
     virtual bool call_run_once_service_io()           { return true; }
     virtual bool request_callback()                   { return true; }
     virtual boost::asio::io_service& get_io_service() { return m_io_service; }