Magic-Folder processing queue Eliot logging fails with a TypeError

[exarkun]

    │   │   │   │   ├── eliot:destination_failure/7/2/2/8/3 2019-03-04 14:24:23
    │   │   │   │   │   ├── exception: exceptions.TypeError
    │   │   │   │   │   ├── message: {u"u'message_type'": u"u'magic-folder:item-queue'", u"u'task_uuid'": u"u'cf81e6ad-fa16-4ee0-b225-45b34592c1ac'", u"u'timestamp'": u'1551709463.704939', u"u'task_level'": u'[7, 2, 2, 8, 2]', u"'items'": u'[<allmydata.frontends.magic_folder.DownloadItem object at 0x7fc647081550>, <allmydata.frontends.magic_folder.DownloadItem object at 0x7fc6470b8c90>, <allmydata.frontends.magic_folder.DownloadItem object at 0x7fc6470b8d50>, <allmydata.frontends.magic_folder.DownloadItem object at 0x7fc6470b8b50>, <allmydata.frontends.magic_folder.DownloadItem object at 0x7fc6470b89d0>]'}

It tries to log DownloadItem (and UploadItem, not pictured here) without providing the proper serializer.

[exarkun]

​https://github.com/tahoe-lafs/tahoe-lafs/pull/560

[GitHub <noreply@…>]

In 2931721/trunk:

Merge pull request #560 from tahoe-lafs/2982.process-queue-logging-fix

Fix serialization errors in the Eliot logging for the queue state.

Fixes: ticket:2982