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_journal.cpp

@@ -184,7 +184,7 @@ journal_t::~journal_t()
     buffer = NULL;
 }
 
-bool journal_t::trim()
+uint64_t journal_t::get_trim_pos()
 {
     auto journal_used_it = used_sectors.lower_bound(used_start);
 #ifdef BLOCKSTORE_DEBUG
@@ -202,26 +202,19 @@ bool journal_t::trim()
         if (journal_used_it == used_sectors.end())
         {
             // Journal is empty
-            used_start = next_free;
+            return next_free;
         }
         else
         {
-            used_start = journal_used_it->first;
-            // next_free does not need updating here
+            // next_free does not need updating during trim
+            return journal_used_it->first;
         }
     }
     else if (journal_used_it->first > used_start)
     {
         // Journal is cleared up to <journal_used_it>
-        used_start = journal_used_it->first;
+        return journal_used_it->first;
     }
-    else
-    {
-        // Can't trim journal
-        return false;
-    }
-#ifdef BLOCKSTORE_DEBUG
-    printf("Journal trimmed to %08lx (next_free=%08lx)\n", used_start, next_free);
-#endif
-    return true;
+    // Can't trim journal
+    return used_start;
 }