Support reloading of the REST API TLS certificates

With MXS-3982 fixed for listeners and servers, the only remaining part is the REST API whose certificates can only be reloaded by restarting MaxScale. The maxctrl reload tls command, added for version 7, could also reload the TLS certificates of the REST API or at least schedule for it to take place.

The hardest part will most likely be the fact that we don't directly control the network layer of the HTTP daemon as libmicrohttpd handles that. There are also some concurrency issues if the websocket interface is being used to stream the logs.