support streaming data form fh to client

support streaming data form fh to client

Use an explicit AnyEvent::Handle similar to websocket proxying.

Needs some special care to make sure we apply backpressure correctly to
avoid caching too much data. Note that because of AnyEvent restrictions,
specifying a "fh" to point to a file or a packet-based socket may result
in unwanted behaviour[0].

[0]: https://metacpan.org/pod/AnyEvent::Handle#DESCRIPTION

Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>