Allow task to be queued with dependencies

[r-ash]

This PR will

• Allow a task to be queued with dependencies using depends_on which takes a vector of task_ids for tasks which need to be complete before this task can be run

Thinking that we can build on this to add

• Function to take id and return any prerequisite tasks we are waiting for (and their status)
• Take id and return any tasks which are dependent on this one - possibly less important
• List tasks in deferred queue
• Workflows (if we want to impl at this level) i.e. define some set of work we want to run and have some wrapper of this queue with depends_on which will do the dance of sending the first to enqueue, getting the ID and then using that in subsequent calls to enqueue

[codecov[bot]]

Codecov Report

Merging #32 (0c9a528) into master (ec3ed6b) will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #32   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           18        19    +1     
  Lines         1153      1237   +84     
=========================================
+ Hits          1153      1237   +84     

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/keys.R	100.00% <100.00%> (ø)
R/rrq_controller.R	100.00% <100.00%> (ø)
R/worker.R	100.00% <100.00%> (ø)
R/worker_run.R	100.00% <100.00%> (ø)

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