Use process_t for managed proxies.

This patch makes the managed proxy subsystem use the process_t data
structure such that we can get events from the PT process while Tor is
running and not just when the PT process is being configured.

See: https://bugs.torproject.org/28179

src/core/mainloop/mainloop.c

@@ -1853,10 +1853,6 @@ second_elapsed_callback(time_t now, const or_options_t *options)
     run_connection_housekeeping(i, now);
   }
 
-  /* 11b. check pending unconfigured managed proxies */
-  if (!net_is_disabled() && pt_proxies_configuration_pending())
-    pt_configure_remaining_proxies();
-
   /* Run again in a second. */
   return 1;
 }