Handle staking info since kaia fork in downloader.

[hyeonLewis]

Proposed changes

After kaia fork, the staking info for block N is determined at block N-1. Also, staking info is no longer stored in DB after kaia fork. This breaks current implementation of downloader to fetch staking info to verify headers (especially in snap sync). In this PR, all necessary staking info for block verification even after kaia fork will be requested to peer and stored in DB temporary. The stored staking info after kaia fork will be deleted after committing blocks to blockchain.

When kaia fork block is N and downloader tries to fetch staking info from N-10 to N+10 when staking interval is 4:

Sync state	Fetched staking info	Stored staking info
While syncing	N-10, N-6, N-2, N-1, N, N+1, ... N+9	All 14 fetched
After sync	-	N-10, N-6, N-2

It enables snap sync which was disabled at #2154.

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