lib/connections: Handle QUIC not being available (#7186)

This does two things:

- Exclude QUIC from go1.16 builds, automatically, for now, since it
  doesn't work and just panics.

- Provide some fake listeners and dialers when QUIC is disabled.

These fake listeners and dialers indicate that they are disabled and
unsupported, which silences "Dialing $address: unknown address scheme:
quic" type of stuff which is not super helpful to the user.

lib/connections/quic_misc.go

@@ -4,7 +4,7 @@
 // License, v. 2.0. If a copy of the MPL was not distributed with this file,
 // You can obtain one at http://mozilla.org/MPL/2.0/.
 
-// +build go1.12,!noquic
+// +build go1.14,!noquic,!go1.16
 
 package connections