Partition: add a local idle_monitor

Make idle events per-partition, but leave Instance::EmitIdle() and its
underlying idle_monitor which broadcasts idle events to all
partitions.

src/client/List.cxx

@@ -34,12 +34,3 @@ ClientList::Remove(Client &client) noexcept
 
 	list.erase(list.iterator_to(client));
 }
-
-void
-ClientList::IdleAdd(unsigned flags) noexcept
-{
-	assert(flags != 0);
-
-	for (auto &client : list)
-		client.IdleAdd(flags);
-}

src/client/List.hxx

@@ -56,8 +56,6 @@ public:
 	}
 
 	void Remove(Client &client) noexcept;
-
-	void IdleAdd(unsigned flags) noexcept;
 };
 
 #endif