Optimize the metrics for redo module for TiCDC

[hongyunyan]

Currently the metrics for redo module is not clear and correct enough to analyze the working situations for redo.
Such as

1. It is impossible to achieve 100% worker busy ratio under the current calculation way.
2. The duration for the flush shows strange and uncorrectly when the duration is quite long.
   ...

To make the metrics more useful and convenient, we need to reorganize the calculation and display of redo metrics.