Refactor get_expected_withdrawals

[jtraglia]

The get_expected_withdrawals is a long, complex function.

This PR splits get_expected_withdrawals into smaller functions:

• get_sweep_withdrawals
• get_pending_partial_withdrawals
• get_builder_withdrawals

Associated with:

• Refactor process_withdrawals #4765

Note: I decided to include prior_withdrawals in all functions so that we don't need create a modified function which makes such a simple change. I didn't include many comments; we can look into making that better. Open to suggestions on other simplifications.

[leolara]

I think this type of change that improves the readability of the specs are very good.

[leolara]

I didn't include many comments; we can look into making that better. Open to suggestions on other simplifications.

Splitting the code in more functions make them more self-explanatory