Allow sequential and parallel batches

[peterneyens]

I made it possible to configure if you want to execute batches to a data source sequentially or in parallel.

By default they will be parallel, so we should mention in the next release notes that people relying on the sequential execution of the batches should update their data sources to specify this sequential execution.

[codecov-io]

Codecov Report

Merging #113 into master will increase coverage by 0.62%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #113      +/-   ##
==========================================
+ Coverage   76.66%   77.29%   +0.62%     
==========================================
  Files          10       10              
  Lines         240      251      +11     
  Branches        2        2              
==========================================
+ Hits          184      194      +10     
- Misses         56       57       +1

Impacted Files	Coverage Δ
shared/src/main/scala/datasource.scala	66.66% <ø> (ø)	✅
shared/src/main/scala/interpreters.scala	93.12% <100%> (-0.21%)	❌
shared/src/main/scala/fetch.scala	81.08% <100%> (ø)	✅

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[ghost]

👏 👏 👏 Amazing work, well done!