This commit enables the prefetch cache per default when rendering configs
and adds the following features:
* prefetch all kinds of Icinga object inheritance (imports)
* prefetch group memberships
* prefetch custom variables
* render vars with the same checksum only once
Roughly measured performance boost with a large real-life config, renders three
times faster as it did before. Might be even more for those suffering from high
latencies when accessing their DB remotely and/or on a slow storage.
fixes#12876
Thomas Gelf