lib-http: Clarify http_client_request_*_payload() API and minor change to it

lib-http: Clarify http_client_request_*_payload() API and minor change to it

The earlier behavior was pretty confusing, and potentially could have caused
double-freeing memory in some situations. Now it's clear that req is set to NULL
always when the request is finished, regardless of whether it has any references left.

Changed http_client_request_finish_payload() to return 0 on success instead of 1.
This could have been left alone, but it's unlikely that there is any code outside
Dovecot core that calls it and this way is cleaner.