- the DeviceMgr no longer uses xpubs to keep track of paired hw devices
- instead, introduce keystore.pairing_code(), based on soft_device_id
- xpubs are now only used in a single place when the actual pairing happens
- motivation is to allow pairing a single device with multiple generic
output script descriptors, not just a single account-level xpub
- as a side-effect, we now allow pairing a device with multiple open
windows simultaneously (if keystores have the same root fingerprint
-- was already the case before if keystores had the same xpub)
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