Describe new delete API and provide trivial implementation

[jamoore]

The new API should look something like:

 Handle delete(Type, Ids, Options);

NB: The initial service implementation will probably delegate to IDelete's existing methods.

[jmoore]

(In [7762]) Initial sketch of an asynchronous Delete API (See #2667)

The implementation does nothing. This is to get the API in place
so that the classes and methods can be improved and tests written.

[jmoore]

(In [7763]) Adding missing implementation for DeleteHandle (See #2667)

[jmoore]

(In [7857]) Nearly complete implementation of IDelete.queueDelete (See #2667)

• Created ThreadPool for background processing (See #2734)
• Added omero.threads.* section to etc/omero.properties
• Added cancel() method to DeleteHandle with a simple state machine
• Added CloseableServant interface to simplify session closing
• Minor refactoring of DeleteBean

[jmoore]

(In [7858]) Adding missing Spring config file to beanRefContext.xml (See #2667)

[jmoore]

(In [7859]) Moving spec.xml to server/resources to be picked up by build (See #2667)

[jmoore]

(In [7868]) Working queueDelete implementation. Time to add more features. (Fix #2667)

[jmoore]

(In [7873]) Correcting issue with DeleteHandle?.cancel() (See #2667)

[jmoore]

(In [7875]) Minor delete improvements (NPEs, logging) (See #2667)

[jmoore]

(In [7877]) Minor corrections to queueDelete and passing integration tests (See #2667, Fix #2668)