[Uptime] Use KVP data structure for synthetics UI state management

[justinkambic]

Today, we use an unordered list to contain and query the synthetic journey data in UI state. Given that each of these entities has a unique identifying value check_group, we should update the code to use a KVP-style data structure.

This came up in code review, but we are electing to add it in a follow-up refactor to keep the amount of review churn to a minimum, and because there's no visible differentiation to the end user as to which of these structures is used. This should still be considered a high priority, with the targeted fix being merged as soon as possible.