Add basic integration tests for WAL delta transfer

[timvisee]

Tracked in: #3477

This adds three basic integration tests for WAL delta transfer, similar to the tests we had for shard snapshot transfer.

The tests include:

• transfer empty WAL delta between two nodes
• transfer WAL delta to recover a failed node, with concurrent upsertions
• transfer WAL delta to empty node, falling back to default transfer method

All Submissions:

• Contributions should target the dev branch. Did you create your branch from dev?
• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

New Feature Submissions:

1. Does your submission pass tests?
2. Have you formatted your code locally using cargo +nightly fmt --all command prior to submission?
3. Have you checked your code using cargo clippy --all --all-features command?