bts: Introduce a PollController that has the responsibility to poll

For each frame indication received by the BTS the poll controller
is asked to expire timedout entries.
diff --git a/src/bts.cpp b/src/bts.cpp
index b278ecf..29cd3e5 100644
--- a/src/bts.cpp
+++ b/src/bts.cpp
@@ -19,6 +19,8 @@
  */
 
 #include <bts.h>
+#include <poll_controller.h>
+
 #include <string.h>
 
 static BTS s_bts;
@@ -40,6 +42,7 @@
 
 BTS::BTS()
 	: m_cur_fn(0)
+	, m_pollController(*this)
 {
 	memset(&m_bts, 0, sizeof(m_bts));
 	m_bts.bts = this;
@@ -48,4 +51,5 @@
 void BTS::set_current_frame_number(int fn)
 {
 	m_cur_fn = fn;
+	m_pollController.expireTimedout(m_cur_fn);
 }