Exception thrown when using NullClient triggering additional unnecessary dynamo calls

![dodge_ball](https://cloud.githubusercontent.com/assets/527874/5168956/128f748a-73fb-11e4-8013-fca882ec30d4.gif)

### Problem

When using the NullClient for brokers that don't use the cache (generally in development), the application was making two calls to the lookup table as the `cache.set` method was throwing an exception due to the wrong number of params passed to the set method. This in turn was then caught in the `rescue` block of the `load` method which then tried to get the lookup location again.

### Solution

Add missing param to `set` method of NullClient.

In this PR we now pass the id to the sequencers `get_last_seen` method, meaning we only have to construct the sequence class once.