Enhancement: Create continuous log files

Previously, when logging was enabled, we started a new log file for every sync.
This worked quite well if you sync a single account and a single folder.
With spaces however we have a multitude of sync folders, which resulted in hundreds of tiny log files.

Now, as soon as a log file's size exceeds 100 MiB, a new log file is started, and the old one is moved and compressed.
The option to delete log files older than 4h was replaced by an option to keep a number of log files.

https://github.com/owncloud/client/issues/9731
