fix(resolver): fix async context leak, gRPC error swallowing, add 48 tests

Fix async resolve goroutine using caller's cancelable context by
switching to context.WithoutCancel. Fix NewGRPCPlugin silently
returning broken resolver on connection failure. Fix "resolover"
typo in log field. Add doc comments to all exported symbols.

resolver/exchanger/exchanger.go

@@ -18,6 +18,7 @@ import (
 	"github.com/miekg/dns"
 )
 
+// Options holds configuration for an Exchanger.
 type Options struct {
 	router    chain.Router
 	tlsConfig *tls.Config
@@ -62,6 +63,7 @@ type Exchanger interface {
 	String() string
 }
 
+// exchanger implements DNS queries over UDP, TCP, TLS (DoT), or HTTPS (DoH).
 type exchanger struct {
 	network string
 	addr    string