Wait for free space again, but count on big_write flushes instead of just flusher activity

src/blockstore_impl.h

@@ -160,6 +160,8 @@ struct __attribute__((__packed__)) dirty_entry
 #define WAIT_JOURNAL 3
 // Suspend operation until the next journal sector buffer is free
 #define WAIT_JOURNAL_BUFFER 4
+// Suspend operation until there is some free space on the data device
+#define WAIT_FREE 5
 
 struct fulfill_read_t
 {
@@ -263,6 +265,7 @@ class blockstore_impl_t
 
     struct journal_t journal;
     journal_flusher_t *flusher;
+    int big_to_flush = 0;
     int write_iodepth = 0;
 
     bool live = false, queue_stall = false;