Improve oEmbed caching. Introduces the concept of a TTL for oEmbed caches and a filter for oembed_ttl.
We will no longer replace previously valid oEmbed responses with an {{unknown}} cache value. When this happens due to reaching a rate limit or a service going down, it is data loss, and is not acceptable. This means that oEmbed caches for a post are no longer deleted indiscriminately every time that post is saved.
oEmbed continues to be cached in post meta, with the addition of a separate meta key containing the timestamp of the last retrieval, which is used to avoid re-requesting a recently cached oEmbed response. By default, we consider a valued cached in the past day to be fresh. This can greatly reduce the number of outbound requests, especially in cases where a post containing multiple embeds is saved frequently.
The TTL used to determine whether or not to request a response can be filtered using oembed_ttl, thus allowing for the possibility of respecting the optional oEmbed response parameter cache_age or altering the period of time a cached value is considered to be fresh.
Now that oEmbeds are previewed in the visual editor as well as the media modal, oEmbed caches are often populated before a post is saved or published. By pre-populating and avoiding having to re-request that response, we also greatly reduce the chances of a stampede happening when a published post is visible before oEmbed caching is complete.
As it previously stood, a stampede was extremely likely to happen, as the AJAX caching was only triggered when $_GET['message'] was 1. The published message is 6. We now trigger the caching every time $_GET['message'] is present on the edit screen, as we are able to avoid triggering so many HTTP requests overall.
props markjaquith. fixes #14759. see #17210.