+ DNS: resolve host names to IP addresses leased by AGH DHCP server

Close #1956

Squashed commit of the following:

commit 21f11632c871e9c17faa77f9cd6a7aa836559779
Author: Simon Zolin <s.zolin@adguard.com>
Date:   Mon Aug 17 19:54:24 2020 +0300

    + DNS: resolve host names to IP addresses leased by AGH DHCP server

dnsforward/dnsforward.go

@@ -51,6 +51,9 @@ type Server struct {
 	stats      stats.Stats
 	access     *accessCtx
 
+	tableHostToIP     map[string]net.IP // "hostname -> IP" table for internal addresses (DHCP)
+	tableHostToIPLock sync.Mutex
+
 	tablePTR     map[string]string // "IP -> hostname" table for reverse lookup
 	tablePTRLock sync.Mutex