Register self generated proposal priority early

Current design treats self generated proposals and the proposals generated by others identically. Priorities can be calculated without generating the block. If a node registers its `Priority` to the `proposal` state variable, it can reject lower priority proposals early to save computation power needed to verify and import blocks.

未指定檔案或測試。首先追蹤 proposal 狀態、Priority 註冊，以及產生、驗證和匯入 block 的路徑。完成的標準是：自行產生的 proposal 提前註冊其 Priority，並且在適當情況下，可以在產生 block 之前拒絕 Priority 較低的 proposal。

由索引模型根據 Issue 內容生成。