Remove partial signatures after finalizing the psbt

[afilini]

Ideally we should remove the partial signatures after we've managed to finalize the tx.

Since now we automatically finalize and don't remove the signatures, we should add two new SignOptions:

• do_not_finalize: just sign without trying to finalize at the end
• do_not_remove_partial_sigs: even if you manage to finalize, leave the sigs there

The current default is (false, true), but maybe we should switch to (false, false) since that's more in line with what BIP174 describes

[kafaichoi]

May I I work on this issue?(probably take 1-2 weeks though as I only has time in weekend)

[afilini]

Sure, no worries about the time, it's not very high priority in my opinion.

I'll assign it to you for now

[notmandatory]

fixed by #621