* global: move to context
Move from golang.org/x/net/context to std lib's context.
Change done with:
for i in $(grep -l '/context' **/*.go); do sed -e 's|golang.org/x/net/context|context|' -i $i; echo $i; done
for i in **/*.go; do goimports -w $i; done
* drop from dns.pb.go as well