Sync only completed writes

Should be a final remaining fix to EC + non-capacitor (non-immediate-commit) write hangs :).

First it was breaking non-EC ("instantly stable") writes because they sometimes
complete out of order which was leading to the following error:

terminate called after throwing an instance of 'std::runtime_error'
  what():  BUG: Unexpected dirty_entry 1000000000001:29480000 v65540 unstable state during flush: 0x151

But it is easily fixed by scanning previous and next dirty_entries in mark_stable.

src/blockstore_sync.cpp

@@ -85,8 +85,7 @@ int blockstore_impl_t::continue_sync(blockstore_op_t *op)
                 left--;
                 auto & dirty_entry = dirty_db.at(sbw);
                 uint64_t dyn_size = dsk.dirty_dyn_size(dirty_entry.offset, dirty_entry.len);
-                if (!space_check.check_available(op, 1, sizeof(journal_entry_big_write) + dyn_size,
-                    (unstable_writes.size()+unstable_unsynced)*journal.block_size))
+                if (!space_check.check_available(op, 1, sizeof(journal_entry_big_write) + dyn_size, 0))
                 {
                     return 0;
                 }