TODO

In -dot mode, if, after the TLS connection may become disconnected, the
redial fails to connect, it results in "operation on closed connection"
errors and a useless connection up until idleTimeout (2 to 4 minutes
later), when the stream ends. For example, see
1-12c59bf6/quad9_dot_1.dnstt.client.log from the 2021-08-02 dnstt-tests
performance measurement:
	2021/08/02 10:00:49 recvLoop: read tcp 10.0.1.2:34788->9.9.9.9:853: read: connection reset by peer
	2021/08/02 10:00:49 sendLoop: write tcp 10.0.1.2:34788->9.9.9.9:853: write: broken pipe
	2021/08/02 10:00:50 tls.Dial: dial tcp 9.9.9.9:853: connect: connection refused
	2021/08/02 10:00:50 recvLoop: read dummy dummy: operation on closed connection
	2021/08/02 10:00:50 send: write dummy dummy: operation on closed connection
	2021/08/02 10:00:52 send: write dummy dummy: operation on closed connection
	...
	2021/08/02 10:03:02 send: write dummy dummy: operation on closed connection
	2021/08/02 10:03:06 send: write dummy dummy: operation on closed connection
	2021/08/02 10:03:09 end stream cde4ab0c:3
To fix this, we could be more persistent in redialing, and/or cause a
redial failure to cause the stream and session to terminate immediately.
We already close TLSPacketConn; we should perhaps also terminate
DNSPacketConn.sendLoop.

Randomize the source port for each query in plain-UDP mode. Currently we
create a socket with net.ListenUDP and use it for all queries, which
means all queries have the same source address. ValdikSS reports that in
regions of Turkmenistan, UDP associations that use the same 4-tuple are
blocked after a few seconds.
https://ntc.party/t/topic/475
My idea for this is to create a new PacketConn abstraction that creates
a bunch (≈100) of goroutines that each do a single query at a time,
creating a new socket for each one and waiting for a response on the
same socket up to a timeout. Similar to HTTPPacketConn and its multiple
instantiations of sendLoop.

Add support for DNS over QUIC
https://www.rfc-editor.org/rfc/rfc9250
https://dnsprivacy.org/wiki/display/DP/DNS+Privacy+Public+Resolvers#DNSPrivacyPublicResolvers-DNS-over-QUIC(DoQ)
https://github.com/AdguardTeam/dnsproxy/blob/660a2da99eb6cc2cf5db9a54f1374dd58fbb5b26/upstream/upstream_quic.go
https://security.googleblog.com/2022/07/dns-over-http3-in-android.html

Externally resolve another domain for the DoH domain, like curl --resolve
E.g. cloudflare.net -> doh.dns.sb