Update powerdns to 3.4.6.

This is a security release fixing CVE-2015-5230.

Bug fixes:
- Avoid superfluous backend recycling
- Removal of dnsdist from the authoritative server distribution
- Add EDNS unknown version handling and tests EDNS unknown version handling

Improvements:
- Update YaHTTP to v0.1.7
- Make trailing/leading spaces stand out in pdnssec check_zone
- GCC 5.2 support and sync boost.m4 macro with upstream
- Log answer packets only if log-dns-details is enabled

(fhajny)