feat: query iterator

[gammazero]

Add a QueryIter function that returns a go iterator that allows ranging over query results. The range yields two values, a result Entry and an error. If an error is returned then iteration stops.

The QueryIterator function is a convenience that helps ensure query results are closed after use and that errors are checked.

Example:

qry := query.Query{
	Prefix: keyPrefix,
}
var foundVal []byte
for ent, err := range QueryIter(dstore, qry) {
	if err != nil {
		return err
	}
	if ent.Key == lookingFor {
		foundVal = ent.Val
		break
	}
}