[net processing] Handle IsContinuationOfLowWorkHeadersSync return value correctly when new headers sync is started

Summary:
This makes TryLowWorkHeadersSync always return true for low work chains, no matter what IsContinuationOfLowWorkHeadersSync returns, preventing potentially commiting low-work headers to disk before the chain of headers has sufficient work.

This is a backport of core#26355 and core#26387

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16787