Pull request: dhcpd: imp static lease validation

Closes #2838.
Updates #2834.

Squashed commit of the following:

commit 608dce28cf6bcbaf5a7f0bf499889ec25777e121
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Thu Mar 18 16:49:20 2021 +0300

    dhcpd: fix windows; imp code

commit 5e56eebf6ab85ca5fd0a0278c312674d921a3077
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Wed Mar 17 18:47:54 2021 +0300

    dhcpd: imp static lease validation

internal/dhcpd/server.go

@@ -11,8 +11,6 @@ type DHCPServer interface {
 	ResetLeases(leases []*Lease)
 	// GetLeases - get leases
 	GetLeases(flags int) []Lease
-	// GetLeasesRef - get reference to leases array
-	GetLeasesRef() []*Lease
 	// AddStaticLease - add a static lease
 	AddStaticLease(lease Lease) error
 	// RemoveStaticLease - remove a static lease
@@ -29,6 +27,8 @@ type DHCPServer interface {
 	Start() error
 	// Stop - stop server
 	Stop()
+
+	getLeasesRef() []*Lease
 }
 
 // V4ServerConf - server configuration
@@ -68,7 +68,12 @@ type V4ServerConf struct {
 	subnetMask net.IPMask    // value for Option SubnetMask
 	options    []dhcpOption
 
-	// Server calls this function when leases data changes
+	// notify is a way to signal to other components that leases have
+	// change.  notify must be called outside of locked sections, since the
+	// clients might want to get the new data.
+	//
+	// TODO(a.garipov): This is utter madness and must be refactored.  It
+	// just begs for deadlock bugs and other nastiness.
 	notify func(uint32)
 }