allow streams to marked as exhausted in more cases

At stream boundaries, we can mark streams as exhausted if there are no
groups active and there are no other ways to report matches. This allows us
to stop maintaining the history buffer on subsequent stream writes.
Previously, streams were only marked as exhausted if a pure highlander case
reported all patterns or the outfix in a sole outfix case died.

src/scratch.h

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2015-2016, Intel Corporation
+ * Copyright (c) 2015-2017, Intel Corporation
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are met:
@@ -73,8 +73,11 @@ struct catchup_pq {
 /** \brief Status flag: user requested termination. */
 #define STATUS_TERMINATED   (1U << 0)
 
-/** \brief Status flag: all possible matches on this stream have
- * been raised (i.e. all its exhaustion keys are on.) */
+/** \brief Status flag: it has been determined that it is not possible for this
+ * stream to raise any more matches.
+ *
+ * This may be because all its exhaustion keys are on or for other reasons
+ * (anchored sections not matching). */
 #define STATUS_EXHAUSTED    (1U << 1)
 
 /** \brief Status flag: Rose requires rebuild as delay literal matched in