Summary:
In `db_stress` profile the vast majority of CPU time is spent acquiring the `SyncPoint` mutex. I mistakenly assumed #3939 had fixed this mutex contention problem by disabling `SyncPoint` processing. But actually the lock was still being acquired just to check whether processing is enabled. We can avoid that overhead by using an atomic to track whether it's enabled.
Closes https://github.com/facebook/rocksdb/pull/3991
Differential Revision: D8393825
Pulled By: ajkr
fbshipit-source-id: 5bc4e3c722ee7304e7a9c2439998c456b05a6897
