http: add info message about memcap

Display a message about http memcap when it is set in config file.

Redmine ticket: https://redmine.openinfosecfoundation.org/issues/1118

PR builds:
- PR build: https://buildbot.suricata-ids.org/builders/regit/builds/114
- PR pcaps: https://buildbot.suricata-ids.org/builders/regit-pcap/builds/53