Amend project load time for each ProjectProvider instance

Close #596.

Before https://github.com/vasily-kirichenko/FSharpVSPowerTools/commit/86e10470432da11d55117bec7be8cee9322dfded, we amended project load time upon any text change. Find All References were slow but there was no weird issue with FCS exceptions.

Now I recover that part but fixing project load time only occurs when `ProjectProvider` instances are created. It means that we may unload projects from FCS caches whenever there are files added to or removed from projects, etc. 

I hope this makes intermittent failures less likely to happen and we still return answers fast enough in most use cases.