Short-circuit `page_on_front` check during site creation

[boonebgorges]

During site initialization, wp_installing() is set to true. Among other things, this toggle disables all caching from get_option(). While I think that this behavior could use a general review (it's legacy behavior from MU that may have been an overly-broad fix for a narrow alloptions problem), there's one specific offender I'd like to consider addressing: page_on_front. generate_rewrite_rules() calls get_option( 'page_on_front' ) several times for each rewrite rule, and each of these calls requires a database read.

Since page_on_front is set to 0 in the default schema, I propose that we short-circuit the check during site initialization (wp_install_defaults()). Something like:

add_filter( 'pre_option_page_on_front', '__return_empty_string' );
$wp_rewrite->init();
$wp_rewrite->flush_rules();
remove_filter( 'pre_option_page_on_front', '__return_empty_string' );

Not terribly elegant, but it can reduce DB I/O by many dozens of reads.