Merge branch 'dm-batchloader-project' into 'master'

Include project in BatchLoader key to prevent returning blobs for the wrong project

Closes gitlab-ee#4325

See merge request gitlab-org/gitlab-ce!15936

(cherry picked from commit f5114187)

6c4f6d37 Include project in BatchLoader key to prevent returning blobs for the wrong project
f871027e Clear BatchLoader after each spec to prevent holding onto records longer than necessary