[MSD benchmark] Dual writer: mode 3

[khaliqgant]

MSD benchmark PR recreated from Martian Code Review Bench.

Source PR: grafana#90045
Dataset: grafana
Original title: Dual writer: mode 3

What is this feature?

Mode 3 will work the following way:

reads (get, list) : only use unified storage
writes (create, update, delete, delete-collection) : write to unified storage and then asynchronously, write to legacy, just as a safe measure.

Why do we need this feature?

[Add a description of the problem the feature is trying to solve.]

Who is this feature for?

[Add information on what kind of user the feature is for.]

Which issue(s) does this PR fix?:

Fixes https://github.com/grafana/search-and-storage-team/issues/53

Special notes for your reviewer:

Please check that:

• It works as expected from a user's perspective.
• If this is a pre-GA feature, it is behind a feature toggle.
• The docs are updated, and if this is a notable improvement, it's added to our What's New doc.