dumpling should handle cases that estimated count are not accurate

[lichunzhu]

Now dumpling computes estimatedChunks through tidb estRows.

	// every chunk would have eventual adjustments
	estimatedChunks := count / conf.Rows
	estimatedStep := (max-min)/estimatedChunks + 1

When table is not created but not used in tidb, estRows will be 10000. Then estimatedChunks will be very large if conf.Rows is not big. Dumpling may dump many sql files which is not expected.