[rpc] refactor: consolidate sendmany and sendtoaddress code

Summary:
The only new behavior is some error codes are changed from -4 to -6.

This is a backport of core#18202

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9993