Fix files external amazon s3

fixes leftover issues found in https://github.com/owncloud/core/issues/11280
- storage id now based on bucket name
- some performance improvements
- better handling of stat for root folder
- log exceptions
- minor typos

cc @icewind1991 @DeepDiver1975 @th3fallen 

needs forward port to master
Just FYI, there is a TODO for that in the doc block.