Merge branch 'stable-2.13'

* stable-2.13:
  Compile against Gerrit 2.13.9
  Remove usage of java.util.Optional

"Remove usage of java.util.Optional" is reverted in this merge, this
commit was only needed on stable-2.13 branch.

Change-Id: I9220a5fdf443f8537aa58b23491c37c6b6b58e79