Bugfixes on bug11243 fix for the not-added cases and tests

 1. The test that adds things to the cache needs to set the clock back so
    that the descriptors it adds are valid.

 2. We split ROUTER_NOT_NEW into ROUTER_TOO_OLD, so that we can
    distinguish "already had it" from "rejected because of old published
    date".

 3. We make extrainfo_insert() return a was_router_added_t, and we
    make its caller use it correctly.  This is probably redundant with
    the extrainfo_is_bogus flag.