Add support for batching requests across unrelated fetches

[jordiolivares]

I noticed that when processing Fetch requests across Fetch.run calls on an HTTP server the fetch requests weren't batched across executions. This means that if we were processing for example 3000 requests simultaneously where each Fetch is for a single ID, we would have 3000 calls to the DataSource.fetch method.

This PR thus adds support for bundling the calls within a given interval in order to produce a single DataSource.batch call instead. This would massively help throughput of HTTP servers at the cost of latency

[jordiolivares]

For some reason the check fails due to an HTTP error even before the code is compiled/tested with this:

HttpError: Resource not accessible by integration

I have no clue why this is failing

[juanpedromoreno]

Thanks so much for your time @jordiolivares .

I've checked locally, and after running sbt ci-test, I get these issues:

[warn] /temp/fetch/fetch/src/main/scala/datasource.scala isn't formatted properly!
[warn] /temp/fetch/fetch/src/main/scala/datasource.scala isn't formatted properly!
[error] 1 files must be formatted
[error] 1 files must be formatted

Running scalafmt, and running ci-test again, I get the following compilation issues:

[error] /Users/juanpedromoreno/temp/fetch/fetch/src/main/scala/datasource.scala:123:32: value groupMap is not a member of List[(I, Callback)]
[error]           val asMap        = x.groupMap(_._1)(_._2)
[error]                                ^
[error] /Users/juanpedromoreno/temp/fetch/fetch/src/main/scala/datasource.scala:123:32: value groupMap is not a member of List[(I, Callback)]
[error]           val asMap        = x.groupMap(_._1)(_._2)
[error]                                ^
https://repo1.maven.org/maven2/org/http4s/http4s-blaze-client_2.12/0.23.3/http4s-blaze-client_2.12-0.23.3.pom
  100.0% [##########] 5.3 KiB (35.9 KiB / s)
[error] /Users/juanpedromoreno/temp/fetch/fetch/src/main/scala/datasource.scala:130:41: type mismatch;
[error]  found   : F[Nothing]
[error]  required: F[A]
[error] Note: Nothing <: A, but type F is invariant in type _.
[error] You may wish to define _ as +_ instead. (SLS 4.5)
[error]           val fiberWork = F.handleError(resultsHaveBeenSent) { ex =>
[error]                                         ^
[error] /Users/juanpedromoreno/temp/fetch/fetch/src/main/scala/datasource.scala:130:41: type mismatch;
[error]  found   : F[Nothing]
[error]  required: F[A]
[error] Note: Nothing <: A, but type F is invariant in type _.
[error] You may wish to define _ as +_ instead. (SLS 4.5)
[error]           val fiberWork = F.handleError(resultsHaveBeenSent) { ex =>
[error]                                         ^

Could you please take a second look? Thanks!

[juanpedromoreno]

In the meantime, I'm taking a look at the CI/CD issue with the PR labeler.

[jordiolivares]

Seems like I've been in Scala 2.13 for so long I had forgotten about Scala 2.12. I've made the compilation fix, so feel free to review it again!

[juanpedromoreno]

Thank so much @jordiolivares ! Great contribution 🙌