This changes the load addr.dat logic so old 'addr' entries are left as-is.
Why: because we saw at least one (and probably 2 or 3) hang-at-startup bugs in rc5 when erasing the old entries.
The result will be larger than necessary addr.dat files for people who upgrade from old releases; I can live with that for this release. The workaround is to just delete addr.dat and let dns or built-in seeding to give you a fresh set of peers.