LabLink 2.2.0 — one client, several benches

One client, several benches. Built and tested against two Raspberry Pis with a
supply on each.

✨ Added

• Connect to several servers at once. The client held exactly one
  connection: connecting to a second Pi replaced the first, and its
  instruments vanished from the Equipment tab until you switched back. A test
  spanning two benches meant working the dropdown. The Equipment and Control
  tabs now fan out over every connected server and merge the results into one
  list, with the bench named on each row -- and only once there is more than
  one server, since on a single-Pi bench that suffix is noise on every row.
• Any instrument can be driven without touching the dropdown. Every
  reading, setpoint, lock and command resolves the connection from the
  selected instrument. Routing by the dropdown instead is the difference
  between setting 12 V on the supply you clicked and setting it on another
  bench.

🐛 Fixed

• Tokens were stored under fixed keys rather than per server, so a second
  login overwrote the first and the next start offered one server's token to
  the other. Single-server use hides this completely. Tokens are now keyed by
  host and port, and an existing unkeyed pair is adopted by the first server
  that asks, so upgrading does not read as a logout.
• Equipment ids are only unique within a server. Two Pis can mint the
  same one -- both test Pis report a 1902B, and the rows are identical apart
  from the bench. Selection and lookups now use a composite key, so the right
  instrument is selected rather than whichever was listed first.
• One unreachable server no longer costs the others. The fan-out is
  concurrent, so three switched-off servers cost one connect timeout rather
  than three; a server that does not answer contributes no rows and a
  tooltip rather than a modal dialog; and a refresh will not start while the
  previous one is still out, since a fan-out can outlast the five-second tab
  refresh.

📝 Changed

• Disconnecting a server disconnects that server, rather than whichever
  connection happened to be active.