Merge branch 'stable-3.9'

* stable-3.9:
  Ignore refs/multi-site/version on global-refdb in replication filter
  Fix Java 8 build and avoid Optional.isEmpty()
  Disable the thread-local refs caching
  Remove the websession-flatfile plugin configuration

Change-Id: I14ee88c72b901512fbb6865d3743ff5d78f81596

.gitignore

diff --git a/.gitignore b/.gitignore
index 8bccbdb..c2375ae 100644
--- a/.gitignore
+++ b/.gitignore
@@ -14,3 +14,6 @@
 
 /eclipse-out/
 setup_local_env/**/*.env
+
+.idea
+*.iml