Fix task wait

[r-ash]

This PR will

• Add some useful groupings of tasks, e.g. TASK$waiting and use these in logic which should help minimise the amount of changes we need to add a new status at any point
• Add a helper for running task cleanup
• Call this when tasks set impossible to make sure task result is saved out so that task_wait works for IMPOSSIBLE tasks
• Add test to check that task_wait works with IMPOSSIBLE

[codecov[bot]]

Codecov Report

Merging #62 (6b44494) into master (8dd8449) will not change coverage. The diff coverage is 100.00%.

:exclamation: Current head 6b44494 differs from pull request most recent head 5a98920. Consider uploading reports for the commit 5a98920 to get more accurate results

@@            Coverage Diff            @@
##            master       #62   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           25        26    +1     
  Lines         1722      1720    -2     
=========================================
- Hits          1722      1720    -2     

Impacted Files	Coverage Δ
R/common.R	100.00% <ø> (ø)
R/bulk.R	100.00% <100.00%> (ø)
R/dependencies.R	100.00% <100.00%> (ø)
R/rrq_controller.R	100.00% <100.00%> (ø)
R/task_cleanup.R	100.00% <100.00%> (ø)
R/worker_run.R	100.00% <100.00%> (ø)

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