Revert "Adjust to changes in Gerrit core"
Revert submission 283559-currentuser-remove-cache-key
Reason for revert: Causes a latency regression for some hosts
Reverted Changes:
I76bfd3ebc:Adjust to changes in Gerrit core
If7ccfd9a4:Remove unused CurrentUser#cacheKey method
I1378ad083:Remove PerThreadCache
Change-Id: Idd9eb9cb91bb531ee75aa2c2cde26a01ceb5ff0a