Reject offsets that are older than the group expiration time

[toddpalino]

Currently, Burrow will accept any offset for storage. Especially in the case of Zookeeper offsets, it's possible to have stale data (topics that are no longer consumed, or old offsets that have been moved over to Kafka) that will get pulled in and cause false alerts.

This PR adds a check into the storage module to reject offsets if they are older than the expire-group configuration. The reasoning is that if the offset is old enough that we would expire a group with just that offset, we shouldn't accept it at all. This should greatly reduce false alerts due to stale Zookeeper data, and it shouldn't impact current offsets at all.

This also updates the tests to support this and properly test min-distance and offset expiration.

Closes #320

[coveralls]

Coverage decreased (-0.05%) to 73.786% when pulling d522778 on toddpalino:drop-old-offsets into b6184ff on linkedin:master.

[coveralls]

Coverage decreased (-0.05%) to 73.786% when pulling 240838c on toddpalino:drop-old-offsets into b6184ff on linkedin:master.