Expand the Progress struct to contain more detailed information

[shuhaowu]

To evaluate performance, we need a few more data points in the Progress struct of Ghostferry. Right now, the candidates are:

1. Amount of time spent while throttled.
2. The current number of actively copying DataIterator goroutines.
3. The number of rows copied per second (both per table and the sum).
4. The cumulative number of rows copied thus far for a particular table.
5. The number of bytes copied per second (both per table and the sum).
6. The cumulative number of bytes copied thus far for a particular table.

For exporting table specific metrics (3-6), we can alter the TableProgress struct.