webproxy: handle unflushed write buffer

webproxy: handle unflushed write buffer

for proxied requests, we usually tear down the proxy connection
immediately when closing the source connection. this is not the correct
course of action for bulk one-way data streams that are proxied, where
the source connection might be closed, but the proxy connection might
still have data in the write buffer that needs to be written out.

push_shutdown already handles this case (closing the socket/FH after it
has been fully drained).

one example for such a proxied data stream is the 'migrate' data for a
remote migration, which gets proxied over a websocket connection.
terminating the proxied connection early makes the target VM crash for
obvious reasons.

Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>