Revert "When WalSndCaughtUp, sleep only in WalSndWaitForWal()."
This reverts commit
421685812290406daea58b78dfab0346eb683bbb. It caused
idle physical walsenders to busy-wait, as reported by Fujii Masao.
Discussion: https://postgr.es/m/
20200417054146.GA1061007@rfd.leadboat.com