OF-2476: Use System Property instead of JiveGlobals
This introduces a small utility class to centralize the configuration of Trunking, and updates the API for using configuration properties. Finally, a new enable/disable flag is added, instead of depending on the non-emptiness of the collection of domains to trunk for.
OF-2476: Trunking/Gateway support
Openfire can be used to accept connections on behalf of remote domains, acting as a 'trunk' or 'gateway' for/to that domain. This feature can be useful when centralized orchestration of data (monitoring, filtering) is needed. This commit adds basic support and documentation for this feature.
To enable the functionality, all participating servers need to have specific configuration. This is explained in documentation that is added in this commit.
