Migrating connections should be faster for status and memory-based balance

[djshow832]

Development Task

Status and memory-based balance define the max migration time (5s or 10s). Ideally, the connections can be migrated in time. However, in my testing, sometimes migrations report 'i/o time' when dialing the new backend. Thus, the connections are not migrated in time.

The migration speed in the latest version is slower than v1.0.0 when the connections are not many. We can speed it up.