Allow using a different query for counting

Problem/Motivation

Fetching every result and counting them in CustomSQLQuery::count() can be quite slow for large queries.

Steps to reproduce

1. Use a slow query with a lot of results in sql_query.
2. Run drush migrate:status.
3. Counting is slow.

Proposed resolution

We could add a sql_count_query option with a different query.
It would allow us to do something like this:

sql_count_query: 'SELECT COUNT(*) FROM foo' # COUNT(*) is usually pretty fast
sql_query: 'SELECT * FROM foo' # Slow query with a lot of results