Change 3 months graphs to 6 months graphs

As of yesterday (cf. legacy/trac#16513 (moved) and legacy/trac#24729 (moved)), we're retaining bandwidth and weights histories on 24 hour granularity for 6 months rather than just 3. In 3 months from today (May 2018) we can extend the 3 months graphs to 6 months graphs with a granularity of 24 hours per data point.

If we decide to do this, the "3_month" string will change to "6_month", and the object will contain twice as many values. We should make sure that Relay Search can handle this before we make this change.

Requires a minor protocol version change.