multi-thread safe getGCSafePointWithRetry

[breezewish]

Enhancement

Currently there is a race condition in getGCSafePointWithRetry which makes it buggy. The getGCSafePointWithRetry are now called in a least 3 kind of threads:

• Read thread
• Background thread
• Manual compaction thread

[breezewish]

We could have two stages to improve this behavior:

Stage 1. Use atomic. This avoids the memory data race, but parallel calls to this function will lead to multiple parallel PD requests.

Stage 2. To be discussed. May be a typical "Single Flight" scenario. We can use similar patterns so that parallel calls (when cache is expired) will not cause multiple PD requests, but wait for the first request instead.