Investigate replacements with our generic dataset

In Snuba we have a concept called `replacements` which is used on the `errors` dataset to enable merging and deleting of issues.

We'd like to support this functionality for issues generated by the issues platform. We need to investigate what would be involved in getting replacements to work on the snuba side in this ticket and implement it.

Will probably involve discussions with search and storage to understand how they work currently. Feel free to use this ticket as an investigation and break out additional tickets with the work needed.

Note that this ticket is mostly intended for the snuba side of this - we need work on the sentry side as well.