Make &broker_store take argument of type string.

It turns out that directly passing a Broker::Store is not really a
bright idea. Because - if we do that - we have to later try to intercept
when the master/clone is generated to figure out what the actual name of
the backing store is.

Turns out that it is much easier to just use the name directly - and
check if a store with that name exists when we want to insert something.

I might want to reconsider this in the future in any case. At the moment
this approach just stores one table into an entire store. In theory, one
store should be able to handle several tables, but... that's more
complex. So let's start with this for now.

src/Val.h

@@ -895,7 +895,7 @@ protected:
 	PrefixTable* subnets;
 	IntrusivePtr<Val> def_val;
 	IntrusivePtr<Expr> change_func;
-	IntrusivePtr<Expr> broker_store;
+	std::string broker_store;
 	// prevent recursion of change functions
 	bool in_change_func = false;