Merge #15463: rpc: Speedup getaddressesbylabel

Summary:
710a7136f9 rpc: Speedup getaddressesbylabel (João Barbosa)

Pull request description:

Fixes #15447. Same approach of #14984, this change avoids duplicate key check when building the JSON response in memory.

ACKs for commit 710a71:

MarcoFalke:
  utACK 710a7136f93133bf256d37dc8c8faf5a6b9ba89d
ryanofsky:
  utACK 710a7136f93133bf256d37dc8c8faf5a6b9ba89d. Just new comments and assert since last review.

Tree-SHA512: 77c95df9ff3793e348619aa070e6fd36df9da1b461d708ab146652cb3699f1a472ef6eb38dafdb8374375cbc97daef07635fcb0501961f167a023309513742e2

Backport of Core PR15463

Similar improvement to D6007

Test Plan:

ninja check check-functional

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6008