Add a server option for the Bluetooth read interval

The server read each Bluetooth device every 60 seconds, and each read is one scan and one connection. An adapter that fails after many connections had no way to get fewer of them.

--bluetooth-interval sets the wait after a good Bluetooth read, and the default stays at 60 seconds. Clients receive the last read during the wait. A failed read still tries again after 60 seconds, or after the interval for a shorter one.

README.md

@@ -95,11 +95,16 @@ device named `all`, not all devices. To include a serial BMS, add a server argum
 such as `-d network:/dev/ttyUSB0`.
 
 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.
+device does not delay the other devices or the clients. 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.
+
+The server reads one Bluetooth device at a time and waits 60 seconds after each
+good Bluetooth read. To change that wait, add `--bluetooth-interval SECONDS` to
+the server command. During the wait, clients receive the last read. After a
+failed Bluetooth read, the server tries again after 60 seconds. An interval
+below 60 seconds also applies after a failed read.
 
 The server answers each client in its own thread, so a slow client does not
 delay the other clients. It closes a connection that stays silent for 5 seconds.