Prototype invoking a post wal write callback in pipelined write mode

[jowlyzhang]

This is a prototype PR. Although it's not planned to be checked in, it would be good to have some reviewing feedback before we send it to Hammerspace for integration and testing.

[jowlyzhang]

How does the user specify the callback? In WriteOptions perhaps?

Yeah, that's a good question. WriteOptions would work, another idea is maybe we can make WriteWithCallback a public API too?

[anand1976]

How does the user specify the callback? In WriteOptions perhaps?

Yeah, that's a good question. WriteOptions would work, another idea is maybe we can make WriteWithCallback a public API too?

Yeah, WriteWithCallback should be fine too. We should have an overload that doesn't take WriteCallback as that seems specific to transaction DB, and maybe name PostWalWriteCallback to something more generic.

[jowlyzhang]

LGTM! Thanks for the PR and quick turnaround

Thank you for the quick review!