trezor: cache whether TrezorBridge is available to speedup scan_devices

If the Bridge is unavailable, on my machine it takes 2 seconds to timeout.
i.e. call_bridge("enumerate") and BridgeTransport.enumerate() both take 2 seconds each.
With this change, if the Bridge is unavailable, DeviceMgr.scan_devices() takes 4 seconds less.
In below log, with 6 different hw devices connected, scan time was originally ~7.5 seconds;
with this change it became ~3.5 seconds.

Now the time is dominated by WebUsbTransport.enumerate(), called by Trezor,
KeepKey, SafeT, ~1.1 seconds each.

-----

I | plugin.DeviceMgr | scan_devices() entered. 1598666278.6756
I | plugin.DeviceMgr | scan_devices(). _scan_devices_with_hid() DONE. 1598666278.7583
I | plugin.DeviceMgr | scan_devices(). starting custom enumeration loop. 1598666278.7593
I | plugin.DeviceMgr | scan_devices(). custom enumeration iter DONE, for <bound method SafeTPlugin.enumerate of <electrum.plugins.safe_t.qt.Plugin object at 0x000001F60060A730>>. 1598666279.9345
I | plugins.trezor.qt.Plugin | trezor custom enumeration entered. 1598666279.9345
I | plugins.trezor.qt.Plugin | trezor custom enumeration. call_bridge('enumerate') DONE. 1598666281.9385
>> trezorlib enumerating <class 'trezorlib.transport.bridge.BridgeTransport'> DONE at 1598666283.9500.
>> trezorlib enumerating <class 'trezorlib.transport.webusb.WebUsbTransport'> DONE at 1598666285.0427.
>> trezorlib enumerating <class 'trezorlib.transport.hid.HidTransport'> DONE at 1598666285.1198.
>> trezorlib enumerating <class 'trezorlib.transport.udp.UdpTransport'> DONE at 1598666285.1237.
I | plugins.trezor.qt.Plugin | trezor custom enumeration. trezorlib.transport.enumerate_devices() DONE. 1598666285.1257
I | plugin.DeviceMgr | scan_devices(). custom enumeration iter DONE, for <bound method TrezorPlugin.enumerate of <electrum.plugins.trezor.qt.Plugin object at 0x000001F60C16F4C0>>. 1598666285.1257
I | plugin.DeviceMgr | scan_devices(). custom enumeration iter DONE, for <bound method KeepKeyPlugin.enumerate of <electrum.plugins.keepkey.qt.Plugin object at 0x000001F60BADF130>>. 1598666286.2251
I | plugin.DeviceMgr | scan_devices(). custom enumeration iter DONE, for <bound method ColdcardPlugin.detect_simulator of <electrum.plugins.coldcard.qt.Plugin object at 0x000001F60BAA5AC0>>. 1598666286.2251
I | plugin.DeviceMgr | scan_devices(). custom enumeration loop DONE. 1598666286.2251
I | plugin.DeviceMgr | scan_devices(). find out what was disconnected DONE. 1598666286.2251
I | plugin.DeviceMgr | scan_devices(). Unpair disconnected devices DONE. 1598666286.2251

electrum/plugins/trezor/trezor.py

@@ -130,6 +130,7 @@ class TrezorPlugin(HW_PluginBase):
         if not self.libraries_available:
             return
         self.device_manager().register_enumerate_func(self.enumerate)
+        self._is_bridge_available = None
 
     def get_library_version(self):
         import trezorlib
@@ -142,17 +143,29 @@ class TrezorPlugin(HW_PluginBase):
         else:
             raise LibraryFoundButUnusable(library_version=version)
 
+    def is_bridge_available(self) -> bool:
+        # Testing whether the Bridge is available can take several seconds
+        # (when it is not), as it is slow to timeout, hence we cache it.
+        if self._is_bridge_available is None:
+            try:
+                call_bridge("enumerate")
+            except Exception:
+                self._is_bridge_available = False
+                # never again try with Bridge due to slow timeout
+                BridgeTransport.ENABLED = False
+            else:
+                self._is_bridge_available = True
+        return self._is_bridge_available
+
     def enumerate(self):
         # If there is a bridge, prefer that.
         # On Windows, the bridge runs as Admin (and Electrum usually does not),
         # so the bridge has better chances of finding devices. see #5420
         # This also avoids duplicate entries.
-        try:
-            call_bridge("enumerate")
-        except Exception:
-            devices = trezorlib.transport.enumerate_devices()
-        else:
+        if self.is_bridge_available():
             devices = BridgeTransport.enumerate()
+        else:
+            devices = trezorlib.transport.enumerate_devices()
         return [Device(path=d.get_path(),
                        interface_number=-1,
                        id_=d.get_path(),