Isolate shared HTTP request functionality

Isolate shared HTTP request functionality

Move shared HTTP request functionality out of http-fetch and http-push,
and replace the two fwrite_buffer/fwrite_buffer_dynamic functions with
one fwrite_buffer function that does dynamic buffering.  Use slot
callbacks to process responses to fetch object transfer requests and
push transfer requests, and put all of http-push into an #ifdef check
for curl multi support.

Signed-off-by: Nick Hengeveld <redacted>
Signed-off-by: Junio C Hamano <redacted>