feat(webconnectivity): long-term-evolution prototype (#882)

See https://github.com/ooni/probe/issues/2237

internal/experiment/webconnectivity/dnscache.go

@@ -0,0 +1,37 @@
+package webconnectivity
+
+import "sync"
+
+// DNSCache wraps a model.Resolver to provide DNS caching.
+//
+// The zero value is invalid; please, use NewDNSCache to construct.
+type DNSCache struct {
+	// mu provides mutual exclusion.
+	mu *sync.Mutex
+
+	// values contains already resolved values.
+	values map[string][]string
+}
+
+// Get gets values from the cache
+func (c *DNSCache) Get(domain string) ([]string, bool) {
+	c.mu.Lock()
+	values, found := c.values[domain]
+	c.mu.Unlock()
+	return values, found
+}
+
+// Set inserts into the cache
+func (c *DNSCache) Set(domain string, values []string) {
+	c.mu.Lock()
+	c.values[domain] = values
+	c.mu.Unlock()
+}
+
+// NewDNSCache creates a new DNSCache instance.
+func NewDNSCache() *DNSCache {
+	return &DNSCache{
+		mu:     &sync.Mutex{},
+		values: map[string][]string{},
+	}
+}