Determine whether to support deleting series

[mattbostock]

The API endpoint to delete series is disabled currently: https://github.com/mattbostock/timbala/blob/8a2f093838778f5e84d1355970ed339eea918c17/internal/api/v1/api.go#L346-L347

Determine whether deleting series is something Timbala should support; doing so has implications for consistency in a distributed system.