Per discussion on #494, this allows external packages (e.g. the upcoming
mtglib/dcprobe for the doctor RPC probe) to reuse the obfuscated2
transport without an internal wrapper.

No public-API change beyond the import path. The only exported names
(Obfuscator, its two methods, and the Secret field) were already
exported within the package.

For a couple of releases we use collected IPs as a prioritized source
for connecting to Telegram. But apparently, they work way worse than it
should, and having connectivity to core ip ALWAYS gives better results.
Thus, this PR flips priorities, so users could have auto-update enabled
as a source of secondary addresses, not primary ones