Enable APC class loading and caching on production environments with APC

It's only enabled if the APC extension is both loaded and enabled.

Signed-off-by: Max Fierke <max@maxfierke.com>
Cache DB metadata, query, and result in APC, if enabled and loaded.

Signed-off-by: Max Fierke <max@maxfierke.com>
Fix stragling 'appsettings' reference.

Signed-off-by: Max Fierke <max@maxfierke.com>
Update code documentation

Signed-off-by: Max Fierke <max@maxfierke.com>