Revert "Remove unused CurrentUser#cacheKey method"

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

java/com/google/gerrit/server/CurrentUser.java

@@ -91,6 +91,12 @@ public abstract class CurrentUser {
    */
   public abstract GroupMembership getEffectiveGroups();
 
+  /**
+   * Returns a unique identifier for this user that is intended to be used as a cache key. Returned
+   * object should to implement {@code equals()} and {@code hashCode()} for effective caching.
+   */
+  public abstract Object getCacheKey();
+
   /** Unique name of the user on this server, if one has been assigned. */
   public Optional<String> getUserName() {
     return Optional.empty();