Parse network-status and bridge-descriptors documents from the bridge authority

See the BridgeDB specification for a full explanation of how these descriptors are used. Right now rdsys only parses the extrainfo documents, but the network-status document is useful for checking the running flag, and either the network-status or bridge-descriptors document is necessary to get the bridge IP address and port for vanilla bridges.

• Evaluate https://github.com/NullHypothesis/zoossh to use in rdsys