fix(pkg/stash): close etcd sessions

Athens creates a new session for each 'Stash' request and then never closes it.
The sessions (etcd leases) are consequently held forever and eventually leads
to resource exhaustion on the etcd cluster.

This has been fixed by closing the acquired session at the end of the request.

Fixes: gomods#1886

pkg/stash/with_etcd.go

@@ -56,12 +56,15 @@ func (s *etcd) Stash(ctx context.Context, mod, ver string) (newVer string, err e
 	const op errors.Op = "etcd.Stash"
 	ctx, span := observ.StartSpan(ctx, op.String())
 	defer span.End()
-	sesh, err := concurrency.NewSession(s.client)
+
+	sess, err := concurrency.NewSession(s.client)
 	if err != nil {
 		return ver, errors.E(op, err)
 	}
+	defer sess.Close()
+
 	mv := config.FmtModVer(mod, ver)
-	mu := concurrency.NewMutex(sesh, mv)
+	mu := concurrency.NewMutex(sess, mv)
 	err = mu.Lock(ctx)
 	if err != nil {
 		return ver, errors.E(op, err)