fixes https://github.com/spesmilo/electrum/issues/7919
In the past, when creating payment requests, we keyed them by on-chain address,
and set/saved the msg of the request as label for the address.
Many places in the code were calling wallet.get_label(addr) with the expectation that
relevant payment requests are found and their message/description (if any) is considered.
wallet.get_label(key) is now made private, and instead the explicit non-polymorphic
wallet.get_label_for_{address,rhash,txid} alternatives should be used.
7b095158bf