Pull request: dhcpd: imp normalization, validation

Updates #3056.

Squashed commit of the following:

commit 875954fc8d59980a39b03032007cbc15d87801ea
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Wed May 5 19:54:24 2021 +0300

    all: imp err msgs

commit c6ea471038ce28f608084b59d3447ff64124260f
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Wed May 5 17:55:12 2021 +0300

    dhcpd: imp normalization, validation

CHANGELOG.md

@@ -17,6 +17,14 @@ and this project adheres to
 ## [v0.106.2] - 2021-05-17 (APPROX.)
 -->
 
+### Fixed
+
+- Uniqueness validation for dynamic DHCP leases ([#3056]).
+
+[#3056]: https://github.com/AdguardTeam/AdGuardHome/issues/3056
+
+
+
 ## [v0.106.1] - 2021-04-30
 
 ### Fixed