Transient cache verify_database_present for 24 hrs

Hi, 

I hope you find this useful; this change eliminates the repeated db queries to verify the database tables present, by storing the results in a transient cache.

* cache both $database_message and  $uninstall_message and reuse if possible, assuming database availability unlikely to change
Hey @adamsilverstein! Thanks for the PR.

While we're at it, might as well switch to using [this static method](https://github.com/adamsilverstein/wp-stream/blob/cache-verify_database_present/includes/network.php#L62) for checking network activation.