Improvement: Only read data valid data from redis.

Currently the l2met outlet will read a list of buckets ids
from a partition set, select bucket ids for which their time
has come, then put back the buckets who are not ripe. This filtering
means that data comes across the wire, is filtered and then must be put
back one-by-one. The filtering is not that big of a deal, it is the putting
back that makes this situation not ideal.

This change modifies the partition data structure in Redis. When buckets
are RPUSHed into Redis, the key (or Id) of that bucket is added to a set.
They key which describes the set now includes a timestamp. E.g.

```
ts.partition.outlet.N
```

The dynamic parts of the key are **ts** and **N**. The UNIX timestamp (ts)
indicates when the bucket will be ready for processing. **N** represents
the partition to which the bucket belongs. The timestamp is computed by the
following formulate:

```
readyAt = truncate(bucketTime + bucketResolution)
```

E.g.
```
bucketTime = 1376285311
bucketResolution = 60
readyAt = 1376285340
```

Finally, it is not clear that this technique will work in all situations.
We are depending on our outlets to never miss a tick. That is, the outlets
must process buckets that are ready potentially every second. More thought
and testing is required before this patch can be trusted.