backend: Rewrite Cosmos DB scanning

In anticipation of the Cosmos DB containers being merged and
partitioned by subscription ID, this commit fully rewrites how
the backend finds and processes operation documents in Cosmos DB.

Instead of querying for all operation documents across all Azure
subscriptions and processing them serially, the backend now uses
a goroutine-based "worker pool" where each worker is responsible
for processing operation documents within a single subscription/
(soon-to-be) Cosmos DB partition.

This dramatically increases the parallelism within the backend
without significantly changing the Cluster Service interaction
logic.

The polling intervals and worker pool size within the backend
may require further tuning.