fix bug: switching the slots and bumping the configEpoch must be executed within the same mutex lock.

[takenliu]

Description

改变slot和提升configEpoch需要在同一个mutex锁下，不然会带来一致性问题。

#451

Motivation and Context

How Has This Been Tested?

Types of Changes

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• Code follows the code style of this project.
• Change requires a change to the documentation.
• I have updated the documentation accordingly.