Revert "SERVER-12650 make syncSourceFeedback's oplogreader effectively single threaded"

This reverts commit 617b4896241649e2a3203e33e5901907790002b5.
SERVER-12650 locking to protect the fallback oplogreader
increase awaitReplication time in slowWeekly/replsets_prefetch_stress.js since it is timing out on windows builders