Router: move segmented netDbs into CCRs.

Router: This moves all client sub-netDbs into the ClientConnectionRunners. This turns the FloodfillNetworkDatabaseSegmentor into a means of looking up netDb's and managing the main netDb and a special netDb for managing multihomes. It adds the ability to use sub-netDbs as a defense against netDb context confusion bugs, and also to switch to a single monolithic netDb if necessary.

Closes #404 (closed) #423 (closed) #420 (closed)