Parallel task execution support

[andrus]

In many cases that I've seen multiple interdependent LmTasks are run sequentially, wrapped in a single job. Users can parallelize these sequences via Java 8 CompletableFuture , still it takes a bit of skill and a bit of code to connect those things together.

Perhaps we can provide an API for users to build a task dependency graph , and let LM figure out parallel/sequential execution flow based on the dependency model. Or should we go full AI and check the extractor target semantics?

[andrus]

One caveat is that LmTask is not a functional interface, so we'll need to figure out how to deal with passing parameters and sync tokens between the stages.

[andrus]

After further analysis, the low hanging fruit here is executing batches of the same task in parallel. Batches are presumably independent from each other.