Fix race condition between set() and push()

[gwynne]

A worker can end up picking up a job between set() and push(), causing a race condition that results in the job running twice. This PR fixes the race by adding an initial state for jobs that haven't yet been pushed.

There is no test included because I couldn't work out a clean way to reliably test for the race.

Fixes #13.

Thanks to @sterien7 for reporting, and to @sidepelican for the excellent analysis of the problem in the original bug report against the old version of the driver!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.54%. Comparing base (4e31dfa) to head (439ab86).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #14      +/-   ##
==========================================
+ Coverage   98.47%   98.54%   +0.07%     
==========================================
  Files           7        7              
  Lines         262      275      +13     
==========================================
+ Hits          258      271      +13     
  Misses          4        4              

Files with missing lines	Coverage Δ
Sources/QueuesFluentDriver/FluentQueue.swift	96.93% <100.00%> (+0.16%)	⬆️
Sources/QueuesFluentDriver/JobModel.swift	100.00% <ø> (ø)

... and 3 files with indirect coverage changes

[sidepelican]

I saw the changes carefully, and I think it's fine.