Replace TaskSequence and @seq_task with SequentialTaskSet

[heyman]

Replace TaskSequence and @seq_task with SequentialTaskSet class that executes tasks in order of declaration.

[codecov]

Codecov Report

Merging #1310 into master will decrease coverage by 0.22%.
The diff coverage is 85.18%.

@@            Coverage Diff             @@
##           master    #1310      +/-   ##
==========================================
- Coverage   80.21%   79.99%   -0.23%     
==========================================
  Files          23       24       +1     
  Lines        2138     2149      +11     
  Branches      322      327       +5     
==========================================
+ Hits         1715     1719       +4     
- Misses        344      346       +2     
- Partials       79       84       +5     

Impacted Files	Coverage Δ
locust/core.py	97.69% <ø> (-1.02%)	⬇️
locust/sequential_taskset.py	85.18% <85.18%> (ø)
locust/runners.py	76.21% <0.00%> (-0.23%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 13d7c3a...223680f. Read the comment docs.