net: Give seednodes time before falling back to fixed seeds

Summary:
Before, we'd remove a seednode from the list right after connecting
to it, leading to a race with loading the fixed seed and connecting
to them.

This is a backport of core#27577

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18368