Proposal: expose SyncPoint API

RocksDB unit tests and stress test use the `SyncPoint` utility heavily to inject delay, failure or custom logic to simulate specific execution sequence. However the facility is only available to rocksdb tests and only available in debug build. If we can expose `SyncPoint` as public API (under /utilities directory), application specific integration test framework can inject delay or error to rocksdb internal to test complex interaction between application behavior and rocksdb behavior. The API can be enabled by a compile flag and it can be enabled under release build.

We would like to work on it but want to know rocksdb team's thoughts beforehand. Thanks.