Stats: Use Transient API to Improve Cache Performance

[mdawaffe]

Fixes #13121

Changes proposed in this Pull Request:

Rather than storing every cached stats item in one big option, store each separately in their own transient.

Moving to transients also means we don't need to worry about expiring the cache ourselves: Core will take care of it.

Bonus: Storing JSON serialized data is significantly more space efficient for our use case than storing data serialized by serialize(). (We're actually doing a bit of both in this PR, but still getting most of the savings.)

Is this a new feature or does it add/remove features to an existing part of Jetpack?

Bug fix.

Testing instructions:

0. git checkout master
1. Load the Jetpack dashboard.
2. wp option get jetpack_restapi_stats_cache - see a bunch of stuff.
3. git checkout fix/13121-use-transients-for-stats-cache
4. HACK: bump JETPACK__VERSION.
5. Load the Jetpack dashboard.
6. wp option get jetpack_restapi_stats_cache - see that the option doesn't exist.
7. wp transient list --search='jetpack_restapi_stats_cache_*' - see the data in the transient(s). Note the expiration.
8. Load the Jetpack dashboard.
9. wp transient list --search='jetpack_restapi_stats_cache_*' - see the data still in the transient(s). See that the expiration has not changed.
10. UNHACK your JETPACK__VERSION.

Proposed changelog entry for your changes:

Improve Stats Cache performance by switching from the WordPress Options API to the WordPress Transient API.

[jetpackbot]

Thank you for the great PR description!

When this PR is ready for review, please apply the [Status] Needs Review label. If you are an a11n, please have someone from your team review the code if possible. The Jetpack team will also review this PR and merge it to be included in the next Jetpack release.

Scheduled Jetpack release: August 6, 2019.
Scheduled code freeze: July 30, 2019

Generated by 🚫 dangerJS against 758a828

[zinigor]

Works great, thank you!

[jeherve]

LGTM! 👍