Fix warnings when using CMake 3.1.0

fixes #7894
Execute checks locally if command_endpoint == local endpoint

fixes #7863
Implement validator for the check_interval attribute

fixes #7591
Use the boost packages from packages.icinga.org
ScriptUtils: Make sure arguments aren't null

fixes #8003
Rename static timer variables
Livestatus: Fix missing host downtimes/comments

fixes #7064
Docs: Explain `nrpe_arguments` in NRPE section

People still use it, and it's the best place there, instead of
the general documentation which should not get messed up with
NRPE at all.

refs #5474
Use relative paths for the OpenSSL DLLs
Fix minor memory leak in the config parser
Make the ThreadPool spawn fewer threads
Fix typeof incorrectly returning null for arrays and dictionaries

fixes #8002
Register type objects as global variables

fixes #8019
Improve output of ToString for type objects

fixes #8020
Redesign validators (WIP)