[ᚬrc/v0.15.z] hotfix rc0.15

[zhangsoledad]

• fix fetch invalid block
• fix response invalid block
• fix repeat process block overwrite block ext

[nervos-bot]

@driftluo is assigned as the chief reviewer

[doitian]

Need a PR to develop as well

[doitian]

error: using '.clone()' on a ref-counted pointer
  --> chain/src/tests/basic.rs:39:24
   |
39 |         .process_block(block.clone(), true)
   |                        ^^^^^^^^^^^^^ help: try this: `Arc::<ckb_core::block::Block>::clone(&block)`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#clone_on_ref_ptr