fix GetSnaphot will return nullptr

[critical27]

What type of PR is this?

• bug
• feature
• enhancement

What problem(s) does this PR solve?

Issue(s) number:

Close #5240

Description:

GetSnaphot would check if the part is leader is not, when it is not, it will return nullptr. It will break the intent of snapshot, because when nullptr is passed to RocksDB, it will read based on latest data instead of the the snapshot we specified. So in this PR, the validation of leadership is removed.

Once use snapshot, user must know exactly what they are doing, because it will definitely read outdate data.

How do you solve it?

Special notes for your reviewer, ex. impact of this fix, design document, etc:

Checklist:

Tests:

• Unit test(positive and negative cases)
• Function test
• Performance test
• N/A

Affects:

• Documentation affected (Please add the label if documentation needs to be modified.)
• Incompatibility (If it breaks the compatibility, please describe it and add the label.）
• If it's needed to cherry-pick (If cherry-pick to some branches is required, please label the destination version(s).)
• Performance impacted: Consumes more CPU/Memory

Release notes:

Please confirm whether to be reflected in release notes and how to describe:

ex. Fixed the bug .....