Merge branch 'stable-2.9' into stable-2.10

* stable-2.9:
  Use US spelling in 2.9.2 release notes
  Release notes for Gerrit 2.9.2
  postgres: fix PK drop, don't assume PK name as TABLE_pkey

Change-Id: I8d07b262cadf7787c6b11b0c70eebc0346f6af14