[nexus] Add service manager to Nexus

[smklein]

Part of #1216

• Adds multiple indices to the database for service querying
• Creates a background "TaskRunner" which periodically executes
  services.
• Within the "TaskRunner", creates a background task for managing
  services.
• Adds a configuration option to disable the background task for tests.

[smklein]

Closing in favor of #1541 , which itself is being split into smaller PRs