Adds a `runNow` api to Task Manager, allowing us to force the refresh of a recurring task.

This PR includes a couple of sustainability changes as well as the feature itself.

1. **Declarative query composition.** At the moment the queries in the TaskStore are huge JSON objects that are hard to maintain and understand. This PR introduces a pattern where the different parts of the query are composed out of type-checked functions, making it easier to maintain and to construct dynamically as needs change. _This was included in this PR as the **markAvailableTasksAsClaimed** query needs different query clauses depending on whether there are specific Tasks we wish to claim first.

2. **Refactoring of the Task Poller** As the `runNow` api is introduced we find Task Manager's lifecycle in a weird state where it has both a _pull_ model, where timeouts & callbacks interact without having to responsd to any external requests, and a _push_ model where requests are made to the new `runNow` api. Balancing these two proved error prone, hard to maintain and had the potential of _lossy_ behaviour where requests are dropped accidentally. To address this TaskPoller has been refactored using Rxjs observables, remodelling the existing _pull_ mechanism as a _push_ mechanism so Task Manager can _respond_ to both _polling_ calls and _runNow_ in a similar fashion.

And ofcourse the main feature of this PR:

3. **runNow api** An api on TaskManager that takes a _task ID_ and attempts to run the task. The call returns a promise which resolves with a result which notifies the caller when the task has either completed successfully, or result in an error.