It's better to have one mechanism for determining whether we're offline
and to improve the SnR of this mechanism over time. We presently have
too many arbitrary heuristics for determining whether we're running in
offline mode. TRTTD is to use polymorphism to eliminate these
conditional checks. (instantiate the node with offline versions of
routing, network, etc.) It'll clean up the core constructor, make it
easier to create ephemeral nodes, and eliminate a class of errors.

@whyrusleeping @jbenet

License: MIT
Signed-off-by: Brian Tiger Chow <brian@perfmode.com>