WIP: initialize EGWeb content loaders

WIP: initialize EGWeb content loaders

This patch adds the ability for EGWeb to call
an initialization function for content loaders
during the child_init phase of Apache backend
startup.

In particular, portions of the R/O object cache
for EGCatLoader are now initialized when a backend
starts up; testing indicates that this can shave
a couple seconds off the time it takes for a
backend to render a bib details page the first time.

Signed-off-by: Galen Charlton <gmc@esilibrary.com>