[io] Execute externally currently requires twisted (async.run)

With the introduction of the new I/O framework, the execution of a job requires to launch the I/O daemon (that will launch and control the user process). This daemon currently needs twisted (async.run) to be executed, while we would ideally prefer not to have such a dependency in beat.core.

To fix this, an optimal decapsulation of the communication via pipes from the I/O daemon would be required, as well as its integration into beat.core.