DIET/MINOR: http: reduce the size of struct http_txn by 8 bytes

DIET/MINOR: http: reduce the size of struct http_txn by 8 bytes

Here again we had some oversized and misaligned entries. The method
and the status don't need 4 bytes each, and there was a hole after
the status that does not exist anymore. That's 8 additional bytes
saved from http_txn and as much for the session.

Also some fields were slightly moved to present better memory access
patterns resulting in a steady 0.5% performance increase.