Cache results of `normalizePath`

`normalizePath` is a rather expensive operation and called multiple times for a single path for every file related operation.

In my development installation with about 9GB of data and 60k files this leads to a performance boost of 24% - in seconds that are 1.86s (!) - for simple searches. With more files the impact will be even more noticeable. Obviously this affects every operation that has in any regard something to do with using OC\Files\Filesystem.

Part of https://github.com/owncloud/core/issues/13221