Automatically setup schedule repairs for each cluster

[sebbonnet]

We've added support for automatically setting scheduled repairs for keyspaces in a cluster - in our local copy. Let me know if this is a feature you'll be interested in, so I can submit the changes.

Features:

• Schedule repairs are automatically created for all keyspaces within a cluster when the cluster is added.
• Cluster keyspaces are periodically checked for changes, so that the scheduled repairs are always in sync with the cluster keyspaces - i.e. scheduled repairs are added for new keyspaces and removed for deleted keyspaces
• Configuration options are available to turn on/off auto scheduling, specify when the scheduling repairs will be activated and how often the keyspaces sync is done
• Schedule repairs are not created for keyspaces with no table.

[Bj0rnen]

Yes, that could be a good feature to have. But leave the default to off, because there can definitely be those keyspaces/tables that have no use for repairs (high throughput, all with a small TTL). Might be useful then to be able to opt out specific keyspaces/tables, but one feature at a time :)

[sebbonnet]

@Bj0rnen here is the PR for it. I've added compatibility with java 1.8 as I've used Duration classes and the various collections goodies available in java8. However I can see that the travis build is failing as it builds using java7. Do you envisage to upgade to java8 anytime soon? If not I'll have to revise the PR to make it work on java7 too