Persist partition|offset|user_id in the keyserver (#1226)

* Persist partition|offset|user_id in the keyserver

Required for a query API which will be used by the syncapi which
will be called when a `/sync` request comes in which will return
a list of user IDs of people who have changed their device keys
between two tokens.

* Add tests and fix maxOffset bug

* s/offset/log_offset/g because 'offset' is a reserved word in postgres

keyserver/storage/storage_test.go

@@ -0,0 +1,57 @@
+package storage
+
+import (
+	"context"
+	"reflect"
+	"testing"
+)
+
+var ctx = context.Background()
+
+func MustNotError(t *testing.T, err error) {
+	t.Helper()
+	if err == nil {
+		return
+	}
+	t.Fatalf("operation failed: %s", err)
+}
+
+func TestKeyChanges(t *testing.T) {
+	db, err := NewDatabase("file::memory:", nil)
+	if err != nil {
+		t.Fatalf("Failed to NewDatabase: %s", err)
+	}
+	MustNotError(t, db.StoreKeyChange(ctx, 0, 0, "@alice:localhost"))
+	MustNotError(t, db.StoreKeyChange(ctx, 0, 1, "@bob:localhost"))
+	MustNotError(t, db.StoreKeyChange(ctx, 0, 2, "@charlie:localhost"))
+	userIDs, latest, err := db.KeyChanges(ctx, 0, 1)
+	if err != nil {
+		t.Fatalf("Failed to KeyChanges: %s", err)
+	}
+	if latest != 2 {
+		t.Fatalf("KeyChanges: got latest=%d want 2", latest)
+	}
+	if !reflect.DeepEqual(userIDs, []string{"@charlie:localhost"}) {
+		t.Fatalf("KeyChanges: wrong user_ids: %v", userIDs)
+	}
+}
+
+func TestKeyChangesNoDupes(t *testing.T) {
+	db, err := NewDatabase("file::memory:", nil)
+	if err != nil {
+		t.Fatalf("Failed to NewDatabase: %s", err)
+	}
+	MustNotError(t, db.StoreKeyChange(ctx, 0, 0, "@alice:localhost"))
+	MustNotError(t, db.StoreKeyChange(ctx, 0, 1, "@alice:localhost"))
+	MustNotError(t, db.StoreKeyChange(ctx, 0, 2, "@alice:localhost"))
+	userIDs, latest, err := db.KeyChanges(ctx, 0, 0)
+	if err != nil {
+		t.Fatalf("Failed to KeyChanges: %s", err)
+	}
+	if latest != 2 {
+		t.Fatalf("KeyChanges: got latest=%d want 2", latest)
+	}
+	if !reflect.DeepEqual(userIDs, []string{"@alice:localhost"}) {
+		t.Fatalf("KeyChanges: wrong user_ids: %v", userIDs)
+	}
+}