Fix data race in dnsfilter

[ameshkov]

The same issue with other global caches there.
We should not use global objects

==================
WARNING: DATA RACE
Write at 0x0000025ac810 by goroutine 36:
  github.com/AdguardTeam/AdGuardHome/dnsfilter.(*Dnsfilter).checkSafeBrowsing()
      adguard-home/dnsfilter/dnsfilter.go:396 +0x693
  github.com/AdguardTeam/AdGuardHome/dnsfilter.(*Dnsfilter).CheckHost()
      adguard-home/dnsfilter/dnsfilter.go:200 +0xc28
  github.com/AdguardTeam/AdGuardHome/dnsforward.(*Server).filterDNSRequest()
      adguard-home/dnsforward/dnsforward.go:502 +0x4ed
  github.com/AdguardTeam/AdGuardHome/dnsforward.(*Server).handleDNSRequest()
      adguard-home/dnsforward/dnsforward.go:445 +0x167
  github.com/AdguardTeam/AdGuardHome/dnsforward.(*Server).handleDNSRequest-fm()
      adguard-home/dnsforward/dnsforward.go:437 +0x8a
  github.com/AdguardTeam/dnsproxy/proxy.(*Proxy).handleDNSRequest()
      /Users/ameshkov/Documents/Work/gopath/pkg/mod/github.com/!adguard!team/dnsproxy@v0.14.0/proxy/proxy.go:914 +0xf1c
  github.com/AdguardTeam/dnsproxy/proxy.(*Proxy).handleUDPPacket()
      /Users/ameshkov/Documents/Work/gopath/pkg/mod/github.com/!adguard!team/dnsproxy@v0.14.0/proxy/proxy.go:625 +0x666
  github.com/AdguardTeam/dnsproxy/proxy.(*Proxy).udpPacketLoop.func1()
      /Users/ameshkov/Documents/Work/gopath/pkg/mod/github.com/!adguard!team/dnsproxy@v0.14.0/proxy/proxy.go:593 +0xe9

Previous read at 0x0000025ac810 by goroutine 37:
  github.com/AdguardTeam/AdGuardHome/dnsfilter.(*Dnsfilter).checkSafeBrowsing()
      adguard-home/dnsfilter/dnsfilter.go:395 +0x343
  github.com/AdguardTeam/AdGuardHome/dnsfilter.(*Dnsfilter).CheckHost()
      adguard-home/dnsfilter/dnsfilter.go:200 +0xc28
  github.com/AdguardTeam/AdGuardHome/dnsforward.(*Server).filterDNSRequest()
      adguard-home/dnsforward/dnsforward.go:502 +0x4ed
  github.com/AdguardTeam/AdGuardHome/dnsforward.(*Server).handleDNSRequest()
      adguard-home/dnsforward/dnsforward.go:445 +0x167
  github.com/AdguardTeam/AdGuardHome/dnsforward.(*Server).handleDNSRequest-fm()
      adguard-home/dnsforward/dnsforward.go:437 +0x8a
  github.com/AdguardTeam/dnsproxy/proxy.(*Proxy).handleDNSRequest()
      /Users/ameshkov/Documents/Work/gopath/pkg/mod/github.com/!adguard!team/dnsproxy@v0.14.0/proxy/proxy.go:914 +0xf1c
  github.com/AdguardTeam/dnsproxy/proxy.(*Proxy).handleUDPPacket()
      /Users/ameshkov/Documents/Work/gopath/pkg/mod/github.com/!adguard!team/dnsproxy@v0.14.0/proxy/proxy.go:625 +0x666
  github.com/AdguardTeam/dnsproxy/proxy.(*Proxy).udpPacketLoop.func1()
      /Users/ameshkov/Documents/Work/gopath/pkg/mod/github.com/!adguard!team/dnsproxy@v0.14.0/proxy/proxy.go:593 +0xe9

Goroutine 36 (running) created at:
  github.com/AdguardTeam/dnsproxy/proxy.(*Proxy).udpPacketLoop()
      /Users/ameshkov/Documents/Work/gopath/pkg/mod/github.com/!adguard!team/dnsproxy@v0.14.0/proxy/proxy.go:592 +0x62b

Goroutine 37 (running) created at:
  github.com/AdguardTeam/dnsproxy/proxy.(*Proxy).udpPacketLoop()
      /Users/ameshkov/Documents/Work/gopath/pkg/mod/github.com/!adguard!team/dnsproxy@v0.14.0/proxy/proxy.go:592 +0x62b

[szolin]

This race is a minor issue actually (it does no harm). We just need to remove all lazy initializations like this, and initialize everything properly in New/Create/Init functions.