invoices: improve perf by caching lnaddr even earlier

During wallet-open, we load all invoices/payreqs. This involved decoding the lnaddrs twice.
Now we only decode once.

For a wallet with ~1000 payreqs, this noticeably sped up wallet-open:
(before:)
8.83 | D | util.profiler | Daemon._load_wallet 6.4317 sec
(after:)
5.69 | D | util.profiler | Daemon._load_wallet 3.4450 sec

It is very expensive to parse all the lnaddrs...

electrum/daemon.py

@@ -463,6 +463,7 @@ class Daemon(Logger):
         return wallet
 
     @staticmethod
+    @profiler
     def _load_wallet(
             path,
             password,

electrum/invoices.py

@@ -196,7 +196,8 @@ class Invoice(StoredObject):
     @lightning_invoice.validator
     def _validate_invoice_str(self, attribute, value):
         if value is not None:
-            lndecode(value)  # this checks the str can be decoded
+            lnaddr = lndecode(value)  # this checks the str can be decoded
+            self.__lnaddr = lnaddr    # save it, just to avoid having to recompute later
 
     @amount_msat.validator
     def _validate_amount(self, attribute, value):