implement DelayQueue

[Krastanov]

based on the example already given in the documentation

I would like to finish this PR before I deal with #94 as this is an important edge case to get correctly.

[Krastanov]

This is hitting a code-coverage miscalculation bug in ResumableFunctions: JuliaDynamics/ResumableFunctions.jl#46

That is why the code coverage looks like it has declined when it is actually tested.

[codecov-commenter]

Codecov Report

Merging #99 (74ef586) into master (91c7e36) will decrease coverage by 1.53%.
The diff coverage is 64.28%.

@@            Coverage Diff             @@
##           master      #99      +/-   ##
==========================================
- Coverage   98.67%   97.15%   -1.53%     
==========================================
  Files          11       12       +1     
  Lines         302      316      +14     
==========================================
+ Hits          298      307       +9     
- Misses          4        9       +5     

Files Changed	Coverage Δ
src/ConcurrentSim.jl	100.00% <ø> (ø)
src/resources/delayed_stores.jl	58.33% <58.33%> (ø)
src/resources/ordered_stores.jl	100.00% <100.00%> (ø)

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[Krastanov]

I have started working on fixing the code coverage issue in JuliaDynamics/ResumableFunctions.jl#71

More needs to be done in ResumableFunctions.jl though. I have asked for advice on the julia slack. Let's see if anything comes out of this.

I will not consider this a blocking issue for this pull request