Add option for automatically updating `medic-sentinel/_local/transitions-seq` after inserting data

[jkuester]

When generating large datasets, it is pretty easy to overwhelm Sentinel with the amount of docs to process. In my experience running locally, with just 500K docs generated into the medic db, the Couch indexing finishes hours before Sentinel catches up. It is not so much that Sentinel is taking too long to run transitions, but really that it is having a hard time even processing the sheer number of new docs.

We should consider possibly adding an option to automatically update the medic-sentinel/_local/transitions-seq value when we are done generating the data. If we set this to the latest changes sequence, then I think that Sentinel will just automatically skip all the inserted docs.

Probably should not have this be the default behavior, though, since in some cases you might want to actually run Sentinel transitions on the new docs...