http: init and cleanup separately from http-walker

http: init and cleanup separately from http-walker

Previously, all our http operations were done with http-walker. With the
new remote-curl helper, we find ourselves using http methods outside of
http-walker - for example, fetching info/refs.

Accomodate this by separating http_init() and http_cleanup() invocations
from http-walker.

Signed-off-by: Tay Ray Chuan <rctay89@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>