Disabling couchstore IO buffering triggers 'vbucket deletion doesn't affect new data' test failure

In a recent gerrit patch I accidentally set IO buffering to always be disabled in CouchKVStore. This resulted in the consistent failure of the 'vbucket deletion doesn't affect new data' test in ep_testsuite_basic when not run under ThreadSanitizer which indicates there is a potential race if disk operations are slowed down / unbuffered.

I did spend some time looking into, but the only significant thing I found is that the couchstore file did not get created on disk before the test started with the buffering disabled.