use more relaxing < and >= ops instead of !=

an example of Search from godocs:

```
package main

import (
	"fmt"
	"sort"
)

func main() {
	a := []int{1, 3, 6, 10, 15, 21, 28, 36, 45, 55}
	x := 6

	i := sort.Search(len(a), func(i int) bool { return a[i] >= x })
	if i < len(a) && a[i] == x {
		fmt.Printf("found %d at index %d in %v\n", x, i, a)
	} else {
		fmt.Printf("%d not found in %v\n", x, a)
	}
}
```

types/validator_set.go

@@ -89,7 +89,7 @@ func (valSet *ValidatorSet) HasAddress(address []byte) bool {
 	idx := sort.Search(len(valSet.Validators), func(i int) bool {
 		return bytes.Compare(address, valSet.Validators[i].Address) <= 0
 	})
-	return idx != len(valSet.Validators) && bytes.Equal(valSet.Validators[idx].Address, address)
+	return idx < len(valSet.Validators) && bytes.Equal(valSet.Validators[idx].Address, address)
 }
 
 // GetByAddress returns an index of the validator with address and validator
@@ -174,7 +174,7 @@ func (valSet *ValidatorSet) Add(val *Validator) (added bool) {
 	idx := sort.Search(len(valSet.Validators), func(i int) bool {
 		return bytes.Compare(val.Address, valSet.Validators[i].Address) <= 0
 	})
-	if idx == len(valSet.Validators) {
+	if idx >= len(valSet.Validators) {
 		valSet.Validators = append(valSet.Validators, val)
 		// Invalidate cache
 		valSet.Proposer = nil
@@ -215,7 +215,7 @@ func (valSet *ValidatorSet) Remove(address []byte) (val *Validator, removed bool
 	idx := sort.Search(len(valSet.Validators), func(i int) bool {
 		return bytes.Compare(address, valSet.Validators[i].Address) <= 0
 	})
-	if idx == len(valSet.Validators) || !bytes.Equal(valSet.Validators[idx].Address, address) {
+	if idx >= len(valSet.Validators) || !bytes.Equal(valSet.Validators[idx].Address, address) {
 		return nil, false
 	}
 	removedVal := valSet.Validators[idx]