Fix journal dirty_start position tracking and some debug prints

Fixes two bugs found during HDD testing :-)
1) OSD crashed with "BUG: Attempt to overwrite used offset of the journal" during
   `fio -bs=900k -iodepth=128` test with 16 MB journal
2) OSD stalled during `fio -bs=512k -iodepth=128` test with 64 MB journal

src/blockstore_flush.h

@@ -103,6 +103,7 @@ public:
     journal_flusher_t(blockstore_impl_t *bs);
     ~journal_flusher_t();
     void loop();
+    bool is_trim_wanted() { return trim_wanted; }
     bool is_active();
     void mark_trim_possible();
     void request_trim();