The original code had a race condition because it never ensured the
standby was caught up before proceeding; add a wait similar to every
other place that does this.

Author: Michaël Paquier
Discussion: https://postgr.es/m/CAB7nPqTm9p+LCm1mVJYvgpwagRK+uibT-pKq0O2-paOWxT62jw@mail.gmail.com