mod_proxy_http: fix load-balancer fallback for requests with a body.
Since r1656259 (or r1656259 in 2.4.41) and the move of prefetch before connect, the balancer fallback case where proxy_http_handler() is re-entered with the next balancer member broke.
We need to save the body (partially) prefetched the first time and reuse it on successive calls, otherwise we might forward partial or empty body.
mod_proxy_http: follow up to r1869216.
Let's call stream_reqbody() for all rb_methods, no RB_SPOOL_CL special case.
This both simplifies code and allows to keep EOS into the input_brigade until it's sent, and thus detect whether we already fetched the whole body if/when proxy_http_handler() re-enters for different balancer members.