feat: overhaul task interface and introduce 'flow ID' for simpler grouping of operations

[garethgeorge]

Significant refactoring of the task interface to decouple it from the orchestrator.

Simultaneously introduces a new concept of a flow ID which is used to group operations in the UI rather than relying on the snapshot ID (which has made for complex handling in the past). Now all operations have a known and immutable flow ID from creation. Any operations belonging to the same flow are explicitly placed in the correct flow as determined either by

• the task that scheduled the operation may ask it's child tasks to inherit the flow ID
• or if a snapshot ID is known the oplog may be queried to find the flow ID for the backup operation that created the snapshot (e.g. to place an indexed snapshot in the same flow as the backup that created it).

This PR also bundles some fixes and reliability improvements that weren't worth forking out into their own commits.