Hdr table free

Hey Andy, I added this in my fork to clear up some memory issues we'd run into at high load. Thought I'd submit a pull request for the canonical repo.

Rationale: Failing client WS handshakes resulted in a double-free of wsi->u.hdr.ah. This change adds lws_free_header_table to wrap the logic for freeing the allocated header table which should prevent future errors.

Ciao!