### What changes were proposed in this pull request?
Tweak the logic of `MemoryManager#currentServingState`
Add Unit Test for this function
```mermaid
graph TB
A(Check Used Memory) --> B{Reach Pause Replicate Threshold}
B --> | N | C{Reach Pause Push Threshold}
B --> | Y | Z(Trigger Pause Push and Replicate)
C --> | N | D{Reach Resume Threshold}
C --> | Y | Y(Trigger Pause Push but Resume Replicate)
D --> | N | E{In Pause Mode}
D --> | Y | X(Trigger Resume Push and Replicate)
E --> | N | U(Do Nothing)
E --> | Y | Y
```
### Why are the changes needed?
Make this method logical, and add unit test to ensure logic won't be accidental modification
### Does this PR introduce _any_ user-facing change?
No
### How was this patch tested?
Add Unit Test
Closes#1811 from zwangsheng/CELEBORN-888.
Authored-by: zwangsheng <2213335496@qq.com>
Signed-off-by: zky.zhoukeyong <zky.zhoukeyong@alibaba-inc.com>
479510cb9c