Move WAIT_RELEASE tbf_state transition to tbf_fsm

While at it, method maybe_start_new_window is renamed to
rcvd_dl_final_ack to make more sense out of the code.

Related: OS#2709
Change-Id: Iebd650c1036ef2d5132789778be7117ce3391c01
diff --git a/src/tbf_fsm.h b/src/tbf_fsm.h
index ea0f58d..1dba80f 100644
--- a/src/tbf_fsm.h
+++ b/src/tbf_fsm.h
@@ -34,6 +34,7 @@
 	TBF_EV_ASSIGN_READY_CCCH, /* TBF Start Time timer triggered */
 	TBF_EV_LAST_DL_DATA_SENT, /* DL TBF sends RLCMAC block containing last DL avilable data buffered */
 	TBF_EV_LAST_UL_DATA_RECVD, /* UL TBF sends RLCMAC block containing last UL data (cv=0) */
+	TBF_EV_FINAL_ACK_RECVD, /* DL ACK/NACK with FINAL_ACK=1 received from MS */
 };
 
 enum tbf_fsm_states {