Distinguish between repeating and unique jobs in job graphs

[andrus]

Currently a config like this will run j2 only once, as graph node uniqueness is determined by the job name only:

g1:
  type: group
  jobs:
     j1:
       dependsOn:
          - j2 
     j3:
       dependsOn:
          - j2:
               params:
                    a: b

We can improve that by making parameters a part of the job "key" (a possible encoding scheme is described in #99).