Security Fix: Decrypt Librato Creds JIT.

Applies to all versions of l2met using the Redis store.

L2met makes HTTP requests to the Librato API on behalf of the
l2met user. L2met does not store the user's Librato credentials
in a long term database. When l2met receives an HTTP request containing
log data, it parses the authorization header, decrypts the header, and
extracts the librato username and password. L2met then keeps the username
and password assoicaited with the log data. This means that the Librato
username and password get passed through Redis on their way to the Librato
outlet and Librato API.

This patch keeps the username and password in their encrypted form
during the Redis phase of the data flow. HTTP requests sent to l2met
are decrypted, but the result is not cached. Instead, the encrypted data
is passed through the system. It is only at the time we make the Librato
HTTP request that we extract the Librato username & password.
prefix metchan metrics with app_name
update metrics file to include metchan changes
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.