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/bitset.go

@@ -0,0 +1,52 @@
+package dhcpd
+
+const bitsPerWord = 64
+
+// bitSet is a sparse bitSet.  A nil *bitSet is an empty bitSet.
+type bitSet struct {
+	words map[uint64]uint64
+}
+
+// newBitSet returns a new bitset.
+func newBitSet() (s *bitSet) {
+	return &bitSet{
+		words: map[uint64]uint64{},
+	}
+}
+
+// isSet returns true if the bit n is set.
+func (s *bitSet) isSet(n uint64) (ok bool) {
+	if s == nil {
+		return false
+	}
+
+	wordIdx := n / bitsPerWord
+	bitIdx := n % bitsPerWord
+
+	var word uint64
+	word, ok = s.words[wordIdx]
+	if !ok {
+		return false
+	}
+
+	return word&(1<<bitIdx) != 0
+}
+
+// set sets or unsets a bit.
+func (s *bitSet) set(n uint64, ok bool) {
+	if s == nil {
+		return
+	}
+
+	wordIdx := n / bitsPerWord
+	bitIdx := n % bitsPerWord
+
+	word := s.words[wordIdx]
+	if ok {
+		word |= 1 << bitIdx
+	} else {
+		word &^= 1 << bitIdx
+	}
+
+	s.words[wordIdx] = word
+}