Add ability to set timeout on index keys in redis

[EmergentBehavior]

I think it would be useful to eventually allow a user to set a default expiry value for non-current index keys. This way, if I don't want to keep all index commits indefinitely, I don't have to manually remove them.

[achambers]

ember-cli-deploy is in the process of merging with ember-deploy. Therefore, no more changes will be made to this repo. Please see the README for more details. /cc @EmergentBehavior