Implement clean shutdown

[flosell]

Currently, when the LambdaCD process is stopped (e.g. through SIGTERM), LambdaCD just stops. This can cause problems:

• Writing the build history might be interrupted, causing corrupted data
• When starting again, builds that were running are still in this state even though they were interrupted when LambdaCD was killed

Tasks:

• make sure no new pipelines are spawned when shutting down
• make sure we can kill running pipelines before shutting down
• make sure we can cleanly shut down the persistence
• add shutdown hook that calls the above process

Optional:

• Provide a way to stop runners/the complete pipeline (see Make runners stoppable #78, Multiple git branches question #76)
• Provide a way to (optionally) wait for pipeline completion before shutting down (implement on demand)

[flosell]

Basic support implemented and released in 0.8.0.

Functions exist to shut down the active parts of LambdaCD and an aggregate and configurable shutdown hook exists as well.
The same hook can also be used to dynamically stop pipelines.

If necessary, it should also be easy to build a feature that, instead of killing active pipelines, waits for them to finish (similar to wait-for-pipelines-to-complete) and only shuts down afterwards.