The current way the trackers are being update is either by adding a new
torrent to the session (on startup or new one), or by changing the
trackers.
This leads to data not being updated in the `torrent.trackers` dict.
To solve this, a cache mechanism was added which will update the dict
on changes or every 5 seconds.
This will help us also get an update regarding `lt.announce_endpoint` in
 each of the trackers.

As part of the changes needed for a more informative trackers tab in the
UIs, we need the info for each of the trackers in the torrent.
For this, new status keys were added, and `tracker_status` is now
considered deprecated.

For tracker info, the keys are:
`trackers_status` - a dict of tracker_url->{status, message}
`trackers_peers` - a dict of tracker_url->number of peers in the tracker

`trackers_status` will contain the status of the torrent and also the
message from the relevant alert.

First, added trackers tab to the WebUI.
Second, now we can view all the trackers and view each:
 * status
 * peers count
 * additional message
Third, moved the private torrent info to the details tab.

closes: https://dev.deluge-torrent.org/ticket/1015