Parallelize adapter functions

[Fra-nk]

Adapters of a similar type (Requirement, Integration, Site...) should be independent of each other. More-so if they are of different types (e.g. communicating with OpenStack and Freiburg), but also if they are of the same type (2 HTCondor adapters).

=> Parallel processing of Spawn/TerminateMachines, Manage, etc.

Possible limitations:

• Events introduce interdependency (may be greatly reduced with Improve MachineRegistry Events propagation #10)
• Socket-limits, time-outs, firewalls
• Thread-safety/locking of logging objects

Python offers a few different options, which have to be explored in more detail:

• Threading
• Processes
• AsyncIO