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,