refactor(sessionresolver): adapt to changing network conditions (#238)

* feat(sessionresolver): try many and use what works

* fix(sessionresolver): make sure we can use quic

* fix: the config struct is unnecessary

* fix: make kvstore optional

* feat: write simple integration test

* feat: start adding tests

* feat: continue writing tests

* fix(sessionresolver): add more unit tests

* fix(sessionresolver): finish adding tests

* refactor(sessionresolver): changes after code review

internal/engine/internal/sessionresolver/childresolver.go

@@ -0,0 +1,27 @@
+package sessionresolver
+
+import (
+	"context"
+	"time"
+)
+
+// childResolver is the DNS client that this package uses
+// to perform individual domain name resolutions.
+type childResolver interface {
+	// LookupHost performs a DNS lookup.
+	LookupHost(ctx context.Context, domain string) ([]string, error)
+
+	// CloseIdleConnections closes idle connections.
+	CloseIdleConnections()
+}
+
+// timeLimitedLookup performs a time-limited lookup using the given re.
+func (r *Resolver) timeLimitedLookup(ctx context.Context, re childResolver, hostname string) ([]string, error) {
+	// Algorithm similar to Firefox TRR2 mode. See:
+	// https://wiki.mozilla.org/Trusted_Recursive_Resolver#DNS-over-HTTPS_Prefs_in_Firefox
+	// We use a higher timeout than Firefox's timeout (1.5s) to be on the safe side
+	// and therefore see to use DoH more often.
+	ctx, cancel := context.WithTimeout(ctx, 4*time.Second)
+	defer cancel()
+	return re.LookupHost(ctx, hostname)
+}