Fix: GC collection on every REST call

[LordHepipud]

Fixes GC collection with Optimize-IcingaForWindowsMemory for every incoming REST connection call

Fixes #491

[haxtibal]

Thanks for the quick response!

I've tested with an almost identical change yesterday, it notably reduced load.

You could go a bit further

• The first GetTotalMemory('forcefullcollection') is costly according to Measure-Command. It's only needed if in debug mode. It's not needed as workaround even if we believe in the PowerShell Garbage Collection bug. Thus 'forcefullcollection' is not required, and the command can be wrapped in an if that checks for debug mode.
• The second GetTotalMemory('forcefullcollection') is only needed if in debug mode or if PS version < 5 (to workaround the said bug). Thus the command can also be made conditional.

And even further:

• also reduce Optimize-IcingaForWindowsMemory calls from REST API worker threads