cli: reduce DefaultRequestHandler memory allocations

Summary:

Simpler code, fewer allocations. No change of behavior. The code has good test coverage in interface_bitcoin_cli.py.

This is a backport of core#19453

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D9988