job/__init__: save number of logic tree samples in uiapi.job_stats table
job/__init__: save number of logic tree samples in uiapi.job_stats table


Former-commit-id: 2d570d6444ddbe0e664b669fa823bf93bc2b2613
gem/oq-engine,gem/oq-engine,gem/oq-engine,gem/oq-engine,gem/oq-engine