Remove creepy "metadata copying" during overwrite

Instead of it, just do not verify checksums of currently mutated objects.
When clean data modification during flush runs in parallel to a read request,
that request may read a mix of old and new data. It may even read a mix of
multiple flushed versions if it lasts too long... And attempts to verify it
using temporary copies of metadata make the algorithm too complex and creepy.

src/blockstore_impl.h

@@ -181,7 +181,7 @@ struct used_clean_obj_t
 {
     int refs;
     uint64_t freed_block; // block+1 if freed, otherwise 0
-    uint8_t *meta; // metadata copy
+    bool was_changed; // was changed by a parallel flush?
 };
 
 // https://github.com/algorithm-ninja/cpp-btree
@@ -352,7 +352,8 @@ class blockstore_impl_t
         iovec *iov, int n_iov, std::function<void(uint32_t, uint32_t, uint32_t)> bad_block_cb);
     bool verify_journal_checksums(uint8_t *csums, uint32_t offset,
         iovec *iov, int n_iov, std::function<void(uint32_t, uint32_t, uint32_t)> bad_block_cb);
-    bool verify_clean_padded_checksums(blockstore_op_t *op, uint64_t clean_loc, uint8_t *csum_buf, iovec *iov, int n_iov);
+    bool verify_clean_padded_checksums(blockstore_op_t *op, uint64_t clean_loc, uint8_t *dyn_data, bool from_journal,
+        iovec *iov, int n_iov, std::function<void(uint32_t, uint32_t, uint32_t)> bad_block_cb);
     int fulfill_read_push(blockstore_op_t *op, void *buf, uint64_t offset, uint64_t len,
         uint32_t item_state, uint64_t item_version);
     void handle_read_event(ring_data_t *data, blockstore_op_t *op);