Fix check in maybeReplaceWithLessBusyConnection

The inequality in maybeReplaceWithLessBusyConnection was supposed
to check if the least busy connection has at least 20% less inflight
requests compared to heavy loaded connection.

However, this check was incorrect:

if alternative == nil || alternative.AvailableStreams() * 120 > c.AvailableStreams() * 100 {
	return c
}

Since "alternative" is the least busy connection, by definition it has
the largest number of available streams. Therefore,
alternative.AvailableStreams() > c.AvailableStreams() is always true.

This commit rewrites the condition by using a number of in use streams
(inflight requests). The inequality now correctly checks is the least
busy connection has at least 20% less in use streams compared to
the heavy loaded connection.

conn.go

@@ -1615,6 +1615,10 @@ func (c *Conn) AvailableStreams() int {
 	return c.streams.Available()
 }
 
+func (c *Conn) InUseStreams() int {
+	return c.streams.InUse()
+}
+
 func (c *Conn) UseKeyspace(keyspace string) error {
 	q := &writeQueryFrame{statement: `USE "` + keyspace + `"`}
 	q.params.consistency = c.session.cons