This adds "stale-while-revalidate" in-memory cache for DNS. Instead of
calling `tokio::net::lookup_host` we use previous result of
`tokio::net::lookup_host` immediately and spawn revalidation task in the
background. This way all lookups after the first successful one return
immediately.
Most of the time results returned by resolvers are the same anyway, but
with this cache we avoid waiting 60 second timeout if DNS request is
lost. Common reason result may be different is round-robin DNS load
balancing and switching from IPv4 to IPv6 network. For round-robin DNS
we don't break load balancing but simply use a different result, and for
IPv6 we anyway likely have a result in persistent cache and can use IPv4
otherwise.
Especially frequent should be the case when you send a message over SMTP
and SMTP connection is stale (older than 60 s), so we open a new one.
With this change new connection will be set up faster as you don't need
to wait for DNS resolution, so message will be sent faster.
This ensures we do not get stuck trying DNS resolver results
when we have a known to work IP address in the cache
and DNS resolver returns garbage
either because it is a captive portal
or if it maliciously wants to get us stuck
trying a long list of unresponsive IP addresses.
This also limits the number of results we try to 10 overall.
If there are more results, we will retry later
with new resolution results.
Otherwise if DNS server returns incorrect results,
we may never try preloaded DNS results.
For example, we may get our first results
from a captive portal.
To test, add `127.0.0.1 example.org`
and try to create an account.
Without this change we only try 127.0.0.1 and fail.
With this change preloaded DNS results are tried as well.
Previously Delta Chat tried all DNS resolution results
in sequence until TCP connection is established successfully,
then tried to establish TLS on top of the TCP connection.
If establishing TLS fails, the whole
connection establishment procedure failed
without trying next DNS resolution results.
In particular, in a scenario
where DNS returns incorrect result
pointing e.g. to a server
that listens on the TCP port
but does not have correpsponding TLS certificate,
Delta Chat now will fall back to the cached result
and connect successfully.
