bankd: terminate bankd connection once map is deleted

This implements a signal based mechanism by which the main thread can
inform worker threads that their mapping has just been removed and
they should hence terminate the connection and return themselves to the
pool.

Change-Id: Id932810d59e9e5d8994629d57aaf180bc96f90f5
diff --git a/src/bankd.h b/src/bankd.h
index 6278a93..f3770f2 100644
--- a/src/bankd.h
+++ b/src/bankd.h
@@ -34,6 +34,8 @@
 	BW_ST_CONN_CLIENT_MAPPED,
 	/* TCP established, client identified, mapping exists, card opened */
 	BW_ST_CONN_CLIENT_MAPPED_CARD,
+	/* TCP established, client identified, but mapping [meanwhile] removed */
+	BW_ST_CONN_CLIENT_UNMAPPED
 };