sba: Create a SBAController that will manage the sbas for a BTS

The PollController is a friend of the SBAController and is allowed
to access the internal list. The list is hidden from everyone else.

This is done because the calculation of timeout should belong into
the PollController and not into the SBAController.
diff --git a/src/poll_controller.cpp b/src/poll_controller.cpp
index 59eef76..461b1a2 100644
--- a/src/poll_controller.cpp
+++ b/src/poll_controller.cpp
@@ -51,11 +51,11 @@
 				gprs_rlcmac_poll_timeout(bts, tbf);
 		}
 	}
-	llist_for_each_entry_safe(sba, sba2, &gprs_rlcmac_sbas, list) {
+	llist_for_each_entry_safe(sba, sba2, &m_bts.sba()->m_sbas, list) {
 		elapsed = (frame_number + 2715648 - sba->fn) % 2715648;
 		if (elapsed >= 20 && elapsed < 2715400) {
 			/* sba will be freed here */
-			gprs_rlcmac_sba_timeout(sba);
+			m_bts.sba()->timeout(sba);
 		}
 	}