Summary:
Fix the following compile warning in db/db_impl

  db/db_impl.cc:1603:19: error: implicit conversion loses integer precision: 'const uint64_t' (aka 'const unsigned long') to 'int' [-Werror,-Wshorten-64-to-32]
     info.job_id = job_id;
                 ~ ^~~~~~

Test Plan: db_test

Reviewers: sdong

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D39423