Add more debugging information

For some time, wallabako no longer deletes the read entries. This MR is not a fix but just trying to figure out why.

2020/02/26 09:00:33 URL https://read.mart-e.be/export/85.epub older than local file /mnt/onboard/wallabako/85.epub, skipped (2020-02-26 08:25:56 +0100 CET > 2017-10-19 13:54:13 +0000 +0000)
2020/02/26 09:00:33 URL https://read.mart-e.be/export/41.epub older than local file /mnt/onboard/wallabako/41.epub, skipped (2020-02-26 08:25:56 +0100 CET > 2017-04-30 11:09:43 +0000 +0000)
2020/02/26 09:00:33 URL https://read.mart-e.be/export/621.epub older than local file /mnt/onboard/wallabako/621.epub, skipped (2020-02-26 08:21:40 +0100 CET > 2020-02-25 19:57:15 +0000 +0000)
2020/02/26 09:00:33 URL https://read.mart-e.be/export/620.epub older than local file /mnt/onboard/wallabako/620.epub, skipped (2020-02-26 08:22:06 +0100 CET > 2020-02-25 14:53:53 +0000 +0000)
2020/02/26 09:00:33 URL https://read.mart-e.be/export/619.epub older than local file /mnt/onboard/wallabako/619.epub, skipped (2020-02-26 08:21:40 +0100 CET > 2020-02-25 14:53:50 +0000 +0000)
2020/02/26 09:00:33 URL https://read.mart-e.be/export/618.epub older than local file /mnt/onboard/wallabako/618.epub, skipped (2020-02-26 08:23:22 +0100 CET > 2020-02-25 14:53:46 +0000 +0000)
2020/02/26 09:00:33 URL https://read.mart-e.be/export/615.epub older than local file /mnt/onboard/wallabako/615.epub, skipped (2020-02-26 08:23:22 +0100 CET > 2020-02-18 09:35:45 +0000 +0000)

It seems that the date of the local file is the date when the file was saved to wallabag (for instance, entry 618 was saved "2020-02-25 14:53" according to my wallabag instance). And marking the file as read did not change the date. The date of the file retrieved from the server is way more recent, looks like the downloaded time (I tried to remove all books and reimport them for testing) but needs to investigate a bit more.

Have you ever encountered this issue?