min-safe-ts: fix MinSafeTS might be set to MaxUint64 permanently

[HuSharp]

related #991
cherry-pick #994

The core question is the getter

• we introduce PD API to not execute go func which for KV request, resulting in not updating safeTSMap.
  • updateMinSafeTS relies on safeTSMap which makes sense(because actually, we can call updateMinSafeTS to kvReuqestUpdater[to indicate func base]).
• And we need to update minsafeTS to make sure when API fails we can fall back to the original way which is by kv request.
• But the core problem is: updateMinSafeTS will return maxUnit64 when the first kv request returns 0 and then although PD API returns correctly[maybe kv is not initialized], TS can not change maxUnit64.
  • to resolve this question, we need to regard maxUnit64 as 0 which means there is an initial state.

[HuSharp]

@JmPotato @nolouch PTAL as well, thx!