Caching dns client

[amanakin]

Hi there! I see, that doh client is cached via sync.Once. But custom dns client is created on every dns request.

WDYT if we will cache dns client too? https://github.com/xvzc/SpoofDPI/blob/ab4d6819c7f8f82fb3f58cbc2ec2ff34af853948/dns/dns.go#L58

Assign me, if you think this is a good idea.