Replace tight loops in clientsForSentry with chained calls

clientsForSentry has a nested pair of forEach loops which very likely will starve the nodejs event loop when a sentry is deemed to be down.  Replace the loops with a chain of N x M calls that iterate through the userIds/clientIds in the same way.

/cc @zendesk/zendesk-radar

### Steps to merge
 - [ ] :+1: of the team

### References
 - Jira link: https://zendesk.atlassian.net/browse/RADAR-435

### Risks
 - Low: clientsForSentry does not respond to sentry down as expected