Bug 980: fix HTTP memory cleanup at shutdown

Buffers in per thread HTTP header, client body and server body storage
would be freed based on the usage indicator instead of the size
indicator.

As the usage indicator (e.g. hsbd_buffers_list_len) could be reset
while leaving the memory untouched for later reuse, the free function
would not iterate over all memory blocks.

Removed DrMemory suppressions as well.

PRscript:
- PR build: https://buildbot.suricata-ids.org/builders/inliniac/builds/149
- PR pcaps: https://buildbot.suricata-ids.org/builders/inliniac-pcap/builds/69
Bug #980.