Read each device in a background thread

A failed JBD read retried forever inside a client request and blocked every other device and client. The server now starts one polling thread for each device, and a request returns the last snapshot. Only the first request after startup waits, until each device completes one read attempt.

One lock makes sure that only one Bluetooth collection runs at a time. A serial BMS retries a failed read after 1 second and keeps its last snapshot for 30 seconds. An error in a collection does not end the polling of that device.

README.md

@@ -94,8 +94,12 @@ Omit `--ups` for all devices. Add `--ups river2` to select one. `--ups all` mean
 device named `all`, not all devices. To include a serial BMS, add a server argument
 such as `-d network:/dev/ttyUSB0`.
 
-The server caches Bluetooth data for 60 seconds. Failed reads wait 60 seconds
-before retrying. Unavailable devices are omitted from the response.
+The server reads each device in its own background thread, so a slow or offline
+device does not delay the other devices or the clients. It reads one Bluetooth
+device at a time and waits 60 seconds after each Bluetooth read, including a
+failed one. It reads a serial BMS every 5 seconds and retries a failed serial
+read after 1 second. Unavailable devices are omitted from the response. Only the
+first request after startup waits, until each device completes one read attempt.
 
 #### Diagnostics