wui: "since" and "announced" columns are confusing

[leif]

The list of storage servers in the gateway's web ui includes two columns containing timestamps, labeled "since" and "announced".

I expected that "announced" would be the most recent time that introducer received an announcement from the storage server, and "since" would be the time that the gateway's current connection to the storage server was opened (or, in the case of a disconnected server, the time that the connection was lost).

Instead, it appears that "announced" is actually the time that the gateway first received the announcement from the introducer (which is the time that the running instance of the gateway first successfully connected to the introducer, except in the less common case of storage servers that the introducer learned about after that time).

It appears that the "since" column is doing what I expected in the case of connected servers (showing when the connection was made) but in the case of disconnected servers it shows the current time (when the page was rendered).

The "announced" column is rarely useful with its current behavior, and perhaps should simply be removed.

The "sent" column is useful for connected servers, as it shows how long they've been connected. For disconnected servers the current behavior is also somewhat useful as a stopgap until #1972 (wui: show time page was rendered) is implemented.

However, for disconnected servers there are two things that would be more useful to know, neither of which are currently displayed: when it became disconnected (or better, when it last responded to communication), and when the last connection attempt was made. (Or is that what the current time actually is? Is it constantly attempting to reconnect?)

Related: #816 (Add ping-all-servers button to welcome page)