Use either prop#241 or prop#259 behaviours when the specs differ.
Use either prop#241 or prop#259 behaviours when the specs differ.

This changes the *_GUARDS_THRESHOLDS to be dynamic in the size of the
total number of entry guards in the network (when conforming to
prop#259), and to be static when following prop#241.  It also adds two
@property methods to client.Client to determine which specification
we're currently attempting to conform to.

nmathewson/guardsim