Prime transient and transient timeout options in get_transient

[peterwilsoncc]

What

Makes use of wp_prime_option_caches() within get_transient() to prime the transient and transient timeout options in a single database call.

For expiring transients, this reduces the number of database queries getting a transient from two to one.

Why

This has minimal impact on sites using a small number of transients. On sites running a plugin that makes heavy use of transients it may have a more significant impact.

Trac ticket: https://core.trac.wordpress.org/ticket/61193

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Props peterwilsoncc, swissspidy.

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[swissspidy]

Good idea!

[peterwilsoncc]

Thanks @swissspidy. I decided overnight I should test for the specific queries that's the purpose of the change. I've pushed the changes to the test suite in eb8200a, the source remains unchanged.

[peterwilsoncc]

Committed r58134 / ea8bc97