Make the usage of the registration bot optional/configurable

[liamsi]

Currently the server can not take registration requests from outside (only through the bot). We might want to make this behavior optional/configurable such that the server can be run without the registration bot in between:

coniks-go/keyserver/server.go

Lines 158 to 170 in 1a0e257

	publicTypes := make(map[int]bool)
	publicTypes[protocol.KeyLookupType] = true
	publicTypes[protocol.KeyLookupInEpochType] = true
	publicTypes[protocol.MonitoringType] = true
	server.waitStop.Add(1)
	go func() {
	server.listenForRequests(publicLn, server.makeHandler(publicTypes))
	server.waitStop.Done()
	}()
	// acceptable types for local connection
	localTypes := make(map[int]bool)
	localTypes[protocol.RegistrationType] = true

(Discussion started here: #86 (comment))

[masomel]

I should add that a case in which a server might not want/need to use the bot is when the server is also the identity provider (i.e. if Twitter ran its own key server for its users). The bot is primarily for key servers that are not first-party key servers, i.e. don't hand out identities in their own namespace, such as Tor Messenger.