Pull request: 2509 type-safety vol.1

Merge in DNS/adguard-home from 2509-type-safety to master

Updates #2509.

Squashed commit of the following:

commit 535968eb7de3a9e0817ddb57bc2320e5c5a55086
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Wed Jan 20 15:06:16 2021 +0300

    dhcpd: fix comments

commit dc79b80381fe7a8ecec6f9659fd23710c9229f59
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Wed Jan 20 14:08:10 2021 +0300

    all: improve docs

commit 156ebf6c9bad95f82cd121f019f3b59b77b18ba6
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Tue Jan 19 17:08:15 2021 +0300

    all: improve JSON encoding and decoding

internal/dhcpd/v6.go

@@ -72,7 +72,10 @@ func (s *v6Server) ResetLeases(ll []*Lease) {
 
 // GetLeases - get current leases
 func (s *v6Server) GetLeases(flags int) []Lease {
-	var result []Lease
+	// The function shouldn't return nil value because zero-length slice
+	// behaves differently in cases like marshalling.  Our front-end also
+	// requires non-nil value in the response.
+	result := []Lease{}
 	s.leasesLock.Lock()
 	for _, lease := range s.leases {
 		if lease.Expiry.Unix() == leaseExpireStatic {