Close minor startup race between main and scheduler threads

Summary:
Don't schedule class PeerManagerImpl's background tasks from its
constructor, but instead do that from a separate method,
StartScheduledTasks(), that can be called later at the end of startup,
after other things, such as the active chain, are initialzed.

Backport of core#22577.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, sdulfari

Reviewed By: #bitcoin_abc, sdulfari

Differential Revision: https://reviews.bitcoinabc.org/D12118