Fix lock contention in ThreadPool::Post.
Config Conversion: Allow to define external global templates referenced by all objects.

tests:

$ ./icinga2_convert_v1_v2.pl -c /etc/icinga/icinga.cfg -o conf/
--hosttmpl="hostbumstitmpl"
--servicetmpl="servicebumstitmpl"
--usertmpl="userbumstitmpl"
--timeperiodtmpl="timeperiodbumstitmpl"
--notificationtmpl="notificationbumstitmpl"
--checkcommandtmpl="checkcommandbumstitmpl"
--eventcommandtmpl="eventcommandbumstitmpl"
--notificationcommandtmpl="notificationcommandbumstitmpl"

refs #4697