Move check-seeds from CI to general seed tools

Summary:
Originally, I put check-seeds with CI scripts because setting a specific threshold for "good" seed
connectivity seemed CI-specific to me. However, after some review (https://reviews.bitcoinabc.org/D4725?vs=on&id=14853#inline-28948)
it became clear that this tool could be more general and not actually have any CI-specific logic.

check-seeds should have been moved to contrib/seeds where it belongs since it has no CI-specific logic and
can be leveraged outside of CI.

Test Plan: Run check-seeds on CI for sanity

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D5314