wallet: make sure payment requests are persisted

Fixes: after adding a payment request, if the process was killed,
the payreq might get lost. In case of using the GUI, neither the
callee nor the caller called wallet.save_db().

Unclear where wallet.save_db() should be called...
Now each method tries to persist their changes by default,
but as an optimisation, the caller can pass write_to_disk=False
e.g. when calling multiple such methods and then call wallet.save_db() itself.

If we had partial writes, which would either rm the need for wallet.save_db()
or at least make it cheaper, this code might get simpler...

related: https://github.com/spesmilo/electrum/pull/6435
related: https://github.com/spesmilo/electrum/issues/4823

electrum/commands.py

@@ -882,14 +882,12 @@ class Commands:
         expiration = int(expiration) if expiration else None
         req = wallet.make_payment_request(addr, amount, memo, expiration)
         wallet.add_payment_request(req)
-        wallet.save_db()
         return wallet.export_request(req)
 
     @command('wn')
     async def add_lightning_request(self, amount, memo='', expiration=3600, wallet: Abstract_Wallet = None):
         amount_sat = int(satoshis(amount))
         key = await wallet.lnworker._add_request_coro(amount_sat, memo, expiration)
-        wallet.save_db()
         return wallet.get_formatted_request(key)
 
     @command('w')
@@ -913,9 +911,7 @@ class Commands:
     @command('w')
     async def rmrequest(self, address, wallet: Abstract_Wallet = None):
         """Remove a payment request"""
-        result = wallet.remove_payment_request(address)
-        wallet.save_db()
-        return result
+        return wallet.remove_payment_request(address)
 
     @command('w')
     async def clear_requests(self, wallet: Abstract_Wallet = None):