[BUG] estimate_count function should include custom where option

[balusarakesh]

Describe the bug

• currently the estimate_count function calculates table rows without including the custom --where option provided by the user
• example: if a table has id column from 1 to 1000 and we want to dump only from 100 to 200, estimates are calculated for the full table. This will have no impact on small tables but can cause serious inefficiencies if the tables have hundreds of millions of rows.
• We dump tables with --where clause and we noticed that job count in the queue is the same no matter the range of columns we are dumping

To Reproduce
Command executed:

• run a mydumper for any table with primary key

What mydumper and myloader version has been used?
mydumper 0.12.1, built against MySQL 5.7.33-36

Expected behavior

• rows are estimated including the --where clause

Environment (please complete the following information):

• all OS types

[balusarakesh]

oh wait...including a where clause to the EXPLAIN SELECT is not gonna work because of this... this could be a feature to provide a way to allow users to dump tables for a specific range on a index