Support tracking parts of task execution asynchronously

[doudou]

The main purpose of this PR is to add support to keep track of asynchronously run operations (through a Promise class) through the tasks, ensuring for instance that a failing promise will lead to a task fault. In order to support debugging, roby-log has now the ability to export timepoints in the CTF format, that can be interpreted by e.g. TraceCompass.

It also adds

• infrastructure support to run feature tests in Cucumber. The integration itself, supported by this, is implemented in https://github.com/rock-core/bundles-cucumber.
• connection to Roby's interface is now done through a single port. Other servers (such as the log server) are started on a random port and one needs to query that port in the main interface.

[doudou]

This has been extensively tested in simulation, and ran the whole yesterday live.

[doudou]

It has one major issue left, that in some conditions Syskit would generate a lot more exceptions than expected. We decided to merge anyways, the fix for that problem will come later today.