Investigate fslock usage

[pront]

This discussion originates from this PR.

Reproduction

1. Checkout this branch.

2. In one terminal run:

cargo run --package vector --bin vector --  -w --config /Users/pavlos.rontidis/CLionProjects/vector/pront_tests/vector/configs/lock.yaml

3. In a second terminal run:

cargo run --package vector --bin vector --  -w --config /Users/pavlos.rontidis/CLionProjects/vector/pront_tests/vector/configs/lock.yaml

Result: Both instances run.

Whereas with fs4 the second instance won't start:

2024-01-16T15:13:20.065037Z ERROR vector::cli: Configuration error. error=Couldn't lock "/Users/pavlos.rontidis/CLionProjects/vector/pront_tests/data_dir/.lock". Error: Resource temporarily unavailable (os error 35)

^ which is the desired behavior.

Note

We might be able to delete the fslock usage altogether after this #19612 is done. So we can repurpose this work to an upgrade plan.