Start refactoring trusted_dir_servers into trusted and fallback lists

Start refactoring trusted_dir_servers into trusted and fallback lists

We use trusted_dir_server_t for two pieces of functionality: a list of
all directory authorities, and a list of initial places to look for
a directory.  With this patch we start to separate those two roles.

There is as of now no actual way to be a fallback directory without being
an authority.