files_external speedup by using locks

This PR shows how to speedup the files_external app by using a lock mechanism and prevent from calling unnecessary API requests.

The lock is currently only implemented for dropbox, but if this is a useable way I think it will be easy to adapt this for the other implementations.

Here are some benchmarks from my local dev system:
Following apps were enabled:
* Deleted files
* External storage support
* Share Files
* Text Editor
* Versions

**With locks**
- Total new folder processing time: 2.9252 seconds.
- Total new file processing time: 3.3561 seconds.
- Total rename processing time: 3.7236 seconds
- Total file upload processing time: 3.9940 seconds.

**Without locks**
- Total new folder processing time: 5.2107 seconds.
- Total new file processing time: 11.0666 seconds.
- Total rename processing time: 4.8185 seconds.
- Total upload processing time: 9.1234 seconds.

Please give me some feedback.

cc @karlitschek @MTGap @DeepDiver1975 @icewind1991