feat: support for incremental basebackup

[hashmap-kz]

• Add support for UPLOAD_MANIFEST.
• Add Manifest, ManifestChecksums, Incremental to BaseBackupOptions struct.
• Add unit-tests covered incremental basebackup and manifest flags support.

Since I've tested locally, I've made a bunch of commits that may be 'squashed' if pull-request will be approved.
Thank you!

[jackc]

LGTM! Thanks!

[jackc]

Hmm... The tests passed before the merge. But there seem to be some failures now. https://github.com/jackc/pglogrepl/actions/runs/20191078774

[hashmap-kz]

Hmm... The tests passed before the merge. But there seem to be some failures now. https://github.com/jackc/pglogrepl/actions/runs/20191078774

Hello. Fixed: #79.

Some tests fail with the message failed to connect to host=127.0.0.1 user=pglogrepl database=pglogrepl: dial error (dial tcp 127.0.0.1:5432: connect: connection refused) even on my fork. After I manually clean up caches, all tests pass. This error is not related to the incremental-bb pull request. I've struggling with that error since creating a fork and triggering tests in GitHub Actions (before making any changes yet).

That's a pull-request branch in my fork: