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: I84965f655d62c258c226ad5d585cee24dea047cc