Fix radix lookup thread safety

Radix lookups would update the tree, leading to threading issues.

Ticket:
https://redmine.openinfosecfoundation.org/issues/1073

Prscript:
- PR build: https://buildbot.suricata-ids.org/builders/inliniac/builds/109
- PR pcaps: https://buildbot.suricata-ids.org/builders/inliniac-pcap/builds/30