Import queue steps

[tomaka]

Sits on top of #3146 ; will be easier to review after #3146 is in
Related to #2416

When importing a bunch of blocks, we now yield the importing future after every single block.
In other words: the OS thread runs the task, which imports a single block, then the task is added to the work queue again. This makes it possible for the executor/scheduler to interleave that with other tasks in between.

I consequently removed the keep_going parameter from import_many_blocks. The caller can now interrupt the operation by just dropping the future. This removes the question of what to return if keep_going returns false.

This makes it possible to add a small delay between each imported block, which at the moment is 0.
For #2416, that delay would be non-zero is browsers, so that we don't eat up all of the CPU and give some space to other tasks.