XTS host/port info should be configurable

The XTS (web services transactions) service needs URL information (host, port, protocol) for
a) constructing self-referencing URLs
b) determining the coordinator service location, when running as a client of a remote coordinator.

In the former case this should default to using information from the web container, except that where the web container supports both http and https this should be selectable. However, the host and port should also be capable of being overridden, as in proxy situations the public and internal host/ports are not the same.

In the latter case it should likewise default to the local coordinator URL constructed as above, but allow an alternative URL to be specified for situations where a remote coordinator is preferred.