utils/stats: delete_job_counters returns num keys
utils/stats: delete_job_counters returns num keys

`delete_job_counters` now returns the number of keys it deletes.


Former-commit-id: 17b3c51f342439082699850591635f7bffeacc11 [formerly 720fd6c33933acee5f91ad95d95f3778f7ab4f3f] [formerly 720fd6c33933acee5f91ad95d95f3778f7ab4f3f [formerly 5b95bcfcf012cdcd1a425289ae450c1183dec631]]
Former-commit-id: cce5a4a262cf2ccbafa38c69b17f0e3b3a683c89
Former-commit-id: aa5f88c88acb11a79d23ea3f79e66e01502c8699
gem/oq-engine,gem/oq-engine,gem/oq-engine,gem/oq-engine,gem/oq-engine