[v1] Move block pool operations to a separate class

[heheda12345]

This PR moves containers of KVCacheBlock in KVCacheManager, including block_pool, free_block_queue, and cached_block_hash_to_block into a new BlockPool class. It is just an API clean up, and doesn't change any behavior of the KVCacheManager.

It is a preparation of the hybrid allocator (#11382) and helps avoid code duplication between the current KVCacheManager and the new KVCacheManager for hybrid model.

This PR is split from #13296

CC @comaniac @WoosukKwon

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[comaniac]

Pretty clean. Should be good to go once comments addressed

[heheda12345]

@comaniac I've updated the PR based on your review. Can you take another look?

[comaniac]

LGTM!