ContextCloserify

This PR propagates the use of ContextCloser to:

- net/muxer/Muxer
- net/service/Service
- net/Network
- core/IpfsNode
- routing/dht/IpfsDht
- routing/dht/ProviderManager

(I'll wait on the daemon until #196 lands)

@whyrusleeping lmk other services you want me to make nicer?