[Core][Bugfix]: fix prefix caching for blockv2

[leiwen83]

Previously, we mistake didn't delete block from _cached_blocks's tracking list when it pop from evictor, which would cause immutable allocator still could get that "cached" block, even it is already being freed into hashless as mutablable allocator did in the middle.

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FIX #xxxx (link existing issues this PR will resolve)

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[leiwen83]

cc @cadedaniel
Could you help review this PR?

[leiwen83]

ping @cadedaniel

[cadedaniel]

LGTM, can you add a comment? will merge after!