http_client: add timeouts for critical connects

http_client: add timeouts for critical connects

Use timeout futures for sections that might hang in certain error
conditions. This is mostly intended to be used as a safeguard, not a
first line of defense - i.e. best-effort avoidance of total hangs.

Not every future used for the HttpClient/H2Client is changed, only those
where a quick response is to be expected. For example, the response
reading futures are left alone, so data transfer is never capped with
timeout, only the initial server connect.

It is also used for upgrading to H2 connections, as that can take a long
time on overloaded servers.

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