rpc: Add window final block height to getchaintxstats #16695

This patch is motivated by the desire to make the output of getchaintxstats more useful and optimized for applications to consume and render the data.

Firstly, this data is already available to the RPC, no additional work is done. Currently additional RPC calls will be needed to look up the height of the final block in the window or the block height that began the window.

By adding the block height of the final block in the window, the JSON is "self-contained" and applications can calculate the exact block height range of the window with no additional RPC requests. For example, a web application which wants to render historical information for getchaintxstats RPC on various window sizes might call the RPC with various window lengths, once per day, and store the JSON results somewhere. Because the final block height of each dataset is included, it's no extra work to determine the exact block window range of each JSON response.

FYI all travis CI tests passed, but one job which uses TSAN timed out with: No output has been received in the last 10m0s, this potentially indicates a stalled build or something wrong with the build itself.

Code review utACK. No opinion on concept.

This code does not seem to actually conflict with 16539, that looks like a false positive from the bot

ACK 9a018e7c475cafd9afa5a6352b5004e43bbdc498, could have minor release note though.

@promag I added a new temporary release note file as I saw documented in the dev notes, please let me know if I did it correctly. This is the addition which I put in the Updated RPCs section:

+- The `getchaintxstats` RPC now returns the additional key of
+  `window_final_block_height`

This is enough:

Updated RPCs
------------

- The `getchaintxstats` RPC now returns the additional key of
  `window_final_block_height`.

which will be manually merged in the final release notes.

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The following sections might be updated with supplementary metadata relevant to reviewers and maintainers.

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Conflicts

No conflicts as of last run.

@leto Could you follow up here with the smaller release notes?

@fanquake Sorry, I didn't understand that as a request. I thought I followed the dev notes exactly, and then @promag summarized them above.

What exactly is being asked?

What exactly is being asked? @leto no problem. Basically we'd just like you to modify the release-notes-16695.md file to only include the text which is relevant to this pull request. i.e only this:

Updated RPCs
------------

- The `getchaintxstats` RPC now returns the additional key of
  `window_final_block_height`.

You can checkout the release notes changes in this PR, #16647, as an example of what I mean.

Just have that in the release note file.

@promag @fanquake ok, just fixed that with a new commit. I can also squash these 3 commits into 1 if that is preferred

just fixed that with a new commit. I can also squash these 3 commits into 1 if that is preferred

Thanks for the quick follow up. Yes please squash your commits.

@fanquake latest push has everything in a single commit. Hopefully Travis likes it

ACK d48c1e837ae1bd08e0f18ad1b57ff72675c3d6ad.

ACK d48c1e837ae1bd08e0f18ad1b57ff72675c3d6ad