JAL-3878 Separate server job handle from task's sub jobs.

JAL-3878 Separate server job handle from task's sub jobs.

The former is represented by WebServiceJobHandle class
which is a immutable container of data identifying the job
on the server. The latter, represented by JobI interface,
stores the current state of the job. In principle, a service
job does not need to be tied to a web service client.