…t#3162)

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Makes an internal buffer that we can re-use when building blocks. This
seems to save on average 75 microseconds per block for osmosis blocks.
(Ranging between 1-2 block parts in the relevant time range) This should
be scaling roughly linear with block size.

This lets us remove one allocation cost per complete block coming in. We
only need to re-allocate on the next "biggest ever seen" block we see.

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- [x] Tests written/updated
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