Improve the FusekiModule interface and lifecycle.

Having used FusekiModules, some usages aren't possible.

The "configure" step is after the DataServiceRegistry has been built and it is effectively immutable. The FusekiServer.Builder is available but all dataset set functions are going to be ignored because the DataServiceRegistry is already built.

Changes:

• Put a prepare step in that is called at the very start of server building.
• Provide some accessors to the builder state, rather than just the "add*" operations, to remove and replace dataset service being built. This will allow full modification, at least by rebuilding a new DataService and inserting the one in progress.

FusekiModules should be considered "subject to change" while the functionality is used for real on early cases. For example, being about to modify the server command line with new arguments. At the moment, configuration can be in the server configuration graph but not he comman dline. (c.f. JENA-2268).