cluster manager: gracefully handle static/cds cluster manager init

[mattklein123]

This change refactors CM init so that we can properly do initialization
across static clusters as well as CDS clusters. The normal init flow
looks like this:

1. Initialize static primary clusters
2. Initialize static secondary clusters
3. Start CDS fetch
4. Initialize CDS primary clusters
5. Initialize CDS secondary clusters

This should now work correectly if any of the phases are absent or
immediately complete (as in the case of static clusters).

fixes #367
fixes #330

[mattklein123]

@lyft/network-team