Improve sync behaviour

[MrStahlfelge]

The connection to Explorer is not always reliable and times out for certain addresses while others work.

It makes sense to improve the data stored in app as following to improve situation when there was a timeout or other exception:

• time of last successful sync is stored and updated for every address
• the addresses are sorted by last sync time ascending for syncing so the oldest states get refreshed first
• the overall information when the last sync was performed shows the oldest address entry
• a display is added to the wallet details list showing the timestamp for the address displayed or the oldest of all for the whole wallet