feat: parallelize IMAP and SMTP connection attempts #5915
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Previously for each connection candidate (essentially host and port pair) after resolving the host to a list of IPs Delta Chat iterated IP addresses one by one. Now for IMAP and SMTP we try up to 5 IP addresses in parallel. We start with one connection and add more connections later. If some connection fails, e.g. we try to connect to IPv6 on IPv4 network and get "Network is unreachable" (ENETUNREACH) error, we replace failed connection with another one immediately.
IPv6 addresses do not get any special priority. IP address prioritization happens in
lookup_host_with_cacheand currently addresses that are known to work recently are returned first.For HTTP there is a follow-up PR #5921 that replaces
reqwestandhyper-utilconnection code with custom connection establishment and direct usage ofhyperclient. Currentlyhyper-utilimplements https://datatracker.ietf.org/doc/html/rfc6555 and tries at most two connection at a time: one IPv6 and one IPv4, with IPv4 attempts delayed by 300 ms on dual-stack hosts if both IPv6 and IPv4 addresses are returned.Closes #5799