Add caching back to User Engagement strategy for 2.5 back-compat

[johnjamesjacoby]

With the move to Engagements and Post (or Term) meta, query caching was removed from the 2.5 Users strategy. This is causing some unintended slowdown (specifically on WordPress.org, and) for sites that are unable to easily run the 2.6 database upgrade.

[johnjamesjacoby]

Add caching, with a few helper methods

[netweb]

Looks good to me, 3292.1.patch​ removes an unused $prefix @param PHPDoc from the previous patch

[johnjamesjacoby]

In 6992:

Engagements: add caching to User strategy, for 2.5 compatibility.

This commit re-adds query caching for FIND_IN_SET queries, fixing long pageloads on sites with large amounts of usermeta that are unable to run the 2.6 database upgrade routine(s).

Props johnjamesjacoby, netweb. Fixes #3292. For 2.6.3, branches/2.6.

[johnjamesjacoby]

In 6993:

Engagements: add caching to User strategy, for 2.5 compatibility.

This commit re-adds query caching for FIND_IN_SET queries, fixing long pageloads on sites with large amounts of usermeta that are unable to run the 2.6 database upgrade routine(s).

Props johnjamesjacoby, netweb. Fixes #3292. For 2.7, trunk.

[johnjamesjacoby]

In 7023:

Engagements: fix bugs with caching implementation in User strategy.

This commit adds the $object_id to the end of the cache key, to avoid collisions with other IDs. It also loosens 2 explicit true comparisons because internal calls to update_meta() up the stack may return an integer.

See #3292. For 2.6.4, branches/2.6.

[johnjamesjacoby]

In 7024:

Engagements: fix bugs with caching implementation in User strategy.

This commit adds the $object_id to the end of the cache key, to avoid collisions with other IDs. It also loosens 2 explicit true comparisons because internal calls to update_meta() up the stack may return an integer.

See #3292. For 2.7, trunk.

[johnjamesjacoby]

In 7025:

Engagements: update cache groups in User strategy.

This commit makes all cache groups the same, and adds an inline comment about zero value object IDs. It also fixes a performance issue with pre-2.6 non-upgraded databases causing FIND_IN_SET queries not to be retrieved from cache correctly.

See #3292. For 2.7, trunk.

[johnjamesjacoby]

In 7026:

Engagements: update cache groups in User strategy.

This commit makes all cache groups the same, and adds an inline comment about zero value object IDs. It also fixes a performance issue with pre-2.6 non-upgraded databases causing FIND_IN_SET queries not to be retrieved from cache correctly.

See #3292. For 2.6.4, branches/2.6.