set_transient calls should have expiration to prevent autoloading

Props @kloon for pointing this one out to me (from [`set_transient()` documentation](https://codex.wordpress.org/Function_Reference/set_transient)):

> NB: transients that never expire are autoloaded, whereas transients with an expiration time are not autoloaded. Consider this when adding transients that may not be needed on every page, and thus do not need to be autoloaded, impacting page performance.

Defaulting to `DAY_IN_SECONDS` constant if there is not expiration value set.

@mikejolley Happy with these values or is there something I'm missing?