Comment 1 for bug 1394101

Quoting the upstream changelog for 3.5.5:
http://www.balabit.com/downloads/files/syslog-ng/sources/3.5.5
"
* Major memory leaks have been fixed in various parts of
  syslog-ng, including but not limited to network and file
  destinations and sources, value-pairs (thus affecting
  mongodb, $(format-json), and others), template handling and
  rewrite rules.

  Some of these leaks only happened during reload, some
  leaked on every message. They have now been plugged.
"

3.5.6 contains further memory leak fixes which weren't included in 3.5.5 "due to a merging mistake".