Tasks are no longer executed in parallel

[forman]

Expected behavior

All tasks started by Cate Desktop should run in parallel. This includes operation invocations, data downloads, workspace manipulations.

Actual behavior

Since Cate 2.0.0.dev8 tasks seem to be executed in sequentially only.

Example: a data download is blocking the execution of a resource deletion and subset generation:

The reason is very likely that we started using Tornado 5 in dev8. While migrating from version 4 to 5, we may have introduced this problem.

Steps to reproduce the problem

1. Open a local dataset
2. Download any dataset from ODP
3. Execute any operation on the local dataset --> this wont start until download complete

Specifications

Cate and Cate Desktop 2.0.0.dev8, all platforms