Blocksconvert suite of tools for converting existing chunks data to blocks

[pstibrany]

What this PR does: This PR is a collection of tools used for conversion of existing chunks data to TSDB blocks.

blocksconvert tool has three targets: scanner, scheduler and builder, each implemented in its own package.

Scanner scans single index table, generates plan files and uploads files to long-term storage for use by scheduler and builder. Each plan file consists of all series and their chunks for single day and single tenant. Only single scanner should need to run.

Scheduler looks for uploaded plan files, and hands them to the builder over gRPC. Only one scheduler should run.

Builder asks scheduler for next plan file to work on, and when it receives one, it starts building the TSDB block based on series/chunks in the plan file. When TSDB block building has finished, builder uploads the block to the storage. Many builders can run at the same time, building different blocks.

Scanner can only scan tables in BigTable at the moment.

Scheduler and builder communicate via files in the object store, and require consistency guarantees: uploaded file needs to be returned by following LIST commands (required by scheduler to avoid sending the same plan file to multiple builders), and uploaded file must be immediately readable by other clients (scheduler uploads .inprogress file, which builder then checks for existence).

Checklist

• Tests updated
• Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

[pracucci]

Wow, such an amazing work! 🎉 I spent 1h and half on this and I haven't been able to get all the nuanced. I put particular attention to scheduler and builder and they LGTM. I have read the code, but haven't verified the scanner implementation with the v9/10/11 indexes because of a lack of time.

[pracucci]

This logic may be slighly simplified moving p := plans[base] before the if and plans[base] = p after the if.

[pstibrany]

We don't have base before the if :(