Refactor p2p_add_connections

Summary:
This makes the code much easier to follow by avoiding a log of p2p_idx manipulations, and make it easier to changes the number of connections in the future without having to update the whole test.

Ref T1696.

Test Plan:

./test/functional/test_runner.py p2p_add_connections

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Maniphest Tasks: T1696

Differential Revision: https://reviews.bitcoinabc.org/D10972