refactor: ProviderManager (#492)

* test: augment TestProviderManager test, add notes of future tests
* refactor(provider-manager): order funcs, update names for consistency, add code docs

providers/provider_set.go

@@ -0,0 +1,34 @@
+package providers
+
+import (
+	"time"
+
+	"github.com/libp2p/go-libp2p-core/peer"
+)
+
+// A providerSet has the list of providers and the time that they were added
+// It is used as an intermediary data struct between what is stored in the datastore
+// and the list of providers that get passed to the consumer of a .GetProviders call
+type providerSet struct {
+	providers []peer.ID
+	set       map[peer.ID]time.Time
+}
+
+func newProviderSet() *providerSet {
+	return &providerSet{
+		set: make(map[peer.ID]time.Time),
+	}
+}
+
+func (ps *providerSet) Add(p peer.ID) {
+	ps.setVal(p, time.Now())
+}
+
+func (ps *providerSet) setVal(p peer.ID, t time.Time) {
+	_, found := ps.set[p]
+	if !found {
+		ps.providers = append(ps.providers, p)
+	}
+
+	ps.set[p] = t
+}