Compress priority messages

Currently, CodeChain interacts with uncompressed `PriorityMessage`s and it increases network cost. We can leverage local computation powers to reduce network cost by compressing `PriorityMessage`s. Selective decompression and decoding will also help filter out wrong data quickly in verification steps.

未指定檔案、測試或進入點。先定位 PriorityMessage 的定義以及解碼它的驗證路徑，然後確定壓縮與選擇性解壓縮的需求；完成的標準是在降低網路成本的同時，仍能在驗證過程中篩除錯誤資料。

由索引模型根據 Issue 內容生成。