MEDIUM: http: move header captures from http_txn to struct stream

MEDIUM: http: move header captures from http_txn to struct stream

The header captures are now general purpose captures since tcp rules
can use them to capture various contents. That removes a dependency
on http_txn that appeared in some sample fetch functions and in the
order by which captures and http_txn were allocated.

Interestingly the reset of the header captures were done at too many
places as http_init_txn() used to do it while it was done previously
in every call place.