Teach Tahoe#1010 about Foolscap#208

This implements the design of Tahoe ticket 1010 with the
Foolscap API changes specified in Foolscap ticket 208.
Specifically we make Tahoe expect to use my changes to Foolscap
in this branch:
https://github.com/david415/foolscap/tree/208-accept-the-null-connection-hints