Implement probe orchestration

[bassosimone]

We need to have a complete implementation of probe orchestration in here. For now, we just have support for getting the test lists. Here's what remains to be done:

• implement the register API
• implement the login API
• implement the update API
• implement the task related APIs
• implement the API expected by mobile apps (see mkall-ios, android-libs)
• remove the orchestra implementation from MK

Note that we can safely remove the orchestra implementation from MK because it is marked as an internal API. Therefore, we can do that without violating any API constraint.

[bassosimone]

I have implemented the APIs we currently use on mobile, plus the APIs required by psiphon as part of making psiphon available to probe-engine (see #89). Since there are no plans ahead to implement the rest of probe-orchestration, it perhaps makes sense to put this into a low priority bucket.

[bassosimone]

It seems we're not sold about implementing the rest of orchestration, and we're leaning towards decoupling orchestration for notification and orchestration for fetching targets, so this issue is less important now.

[bassosimone]

We're not going to implement more of probe orchestration. We're going to use Countly for sending notifications to mobile users. So we can safely close this issue.