Add option to disable multiple overwrites of the same journal sector

This makes sense for some SSDs like Intel D3-4510 because they don't
like overwrites of the same sector:

$ fio -direct=1 -rw=write -bs=4k -size=4k -loops=100000 -iodepth=1
  write: IOPS=3142, BW=12.3MiB/s (12.9MB/s)(97.9MiB/7977msec)

$ fio -direct=1 -rw=write -bs=4k -size=128k -loops=100000 -iodepth=1
  write: IOPS=20.8k, BW=81.4MiB/s (85.3MB/s)(543MiB/6675msec)

blockstore_journal.h

@@ -129,6 +129,7 @@ struct journal_sector_info_t
 {
     uint64_t offset;
     uint64_t usage_count;
+    bool written;
     bool dirty;
 };
 
@@ -153,6 +154,7 @@ struct journal_t
     void *sector_buf = NULL;
     journal_sector_info_t *sector_info = NULL;
     uint64_t sector_count;
+    bool no_same_sector_overwrites = false;
     int cur_sector = 0;
     int in_sector_pos = 0;