accounts: let storeAccount set the LastUpdate timestamp

In later commits, we will use this `storeAccount` helper quite often.
Instead of needing to remember to update the timestamp outside the call,
it make sense to instead update the timestamp within the function. Yes
this does mean that sometimes we make no overall changes but do update
the timestamp but this is a pretty standard pattern that a "last
updated" timestamp is updated at any point that we re-write a record
(even if it does not have a net change).

Author: ellemouton

accounts/store_kvdb.go

@@ -145,7 +145,6 @@ func (s *BoltStore) NewAccount(ctx context.Context, balance lnwire.MilliSatoshi,
 		InitialBalance: balance,
 		CurrentBalance: int64(balance),
 		ExpirationDate: expirationDate,
-		LastUpdate:     time.Now(),
 		Invoices:       make(AccountInvoices),
 		Payments:       make(AccountPayments),
 		Label:          label,
@@ -190,7 +189,6 @@ func (s *BoltStore) UpdateAccount(_ context.Context,
 			return ErrAccountBucketNotFound
 		}
 
-		account.LastUpdate = time.Now()
 		return storeAccount(bucket, account)
 	}, func() {})
 }
@@ -200,6 +198,8 @@ func (s *BoltStore) UpdateAccount(_ context.Context,
 func storeAccount(accountBucket kvdb.RwBucket,
 	account *OffChainBalanceAccount) error {
 
+	account.LastUpdate = time.Now()
+
 	accountBinary, err := serializeAccount(account)
 	if err != nil {
 		return err