4904: Eliminate downtime switching collections.

4904: Eliminate downtime switching collections.

* Now re-uses CollectionDirectory and loads new collection contents instead of
unmounting and remounting.  Now sends sighup instead of restarting Docker if
the image is the same.  Almost no downtime switching collections unless Docker
image changes as well.

* Refactor into functions instead executing directly as a module.

* Use arvados.arv-web logger