Fix journal trimming

1) Update journal's used_start in memory only after updating journal superblock.
Doing the opposite is incorrect because part of the journal will be lost if writers
overwrite its old beginning.

2) Sync journal device after updating the superblock.

3) Do not trim in rollback and init because trimming there would also require
updating the superblock. And the only reason to trim in both those places was
to unblock writers. And a guaranteed unblocking method will follow in the next
commit :)

blockstore_flush.h

@@ -59,6 +59,8 @@ class journal_flusher_co
     uint64_t clean_bitmap_offset, clean_bitmap_len;
     void *new_clean_bitmap;
 
+    uint64_t new_trim_pos;
+
     // local: scan_dirty()
     uint64_t offset, end_offset, submit_offset, submit_len;
 
@@ -85,6 +87,7 @@ class journal_flusher_t
     friend class journal_flusher_co;
 
     int journal_trim_counter, journal_trim_interval;
+    bool trimming;
     void* journal_superblock;
 
     int active_flushers;