Attempt journal trim even without new flushes

This is the new guaranteed unblocking method which replaces old trims
in init and rollback, and also fixes a possible stall when just several
writes in the beginning of the journal are flushed without triggering
a subsequent trim.

blockstore_rollback.cpp

@@ -148,6 +148,7 @@ resume_5:
     {
         mark_rolled_back(*v);
     }
+    flusher->mark_trim_possible();
     // Acknowledge op
     op->retval = 0;
     FINISH_OP(op);