Deep copy causes memory flare on unspill

[crusaderky]

• Twin of Unnecessary deep copy causes memory flare on network comms #5107

When spilling a key with pickle5 buffers (e.g. numpy or pandas), you'll get n+2 frames, with the first two being the header and non-buffer pickle output.
This data is written contiguously to a file.

When unspilling the key, the pickle5 buffers are deep-copied by deserialization; this is both time-consuming (#7351 (comment)) and causes a memory flare.

Note that this issue only affects uncompressible data (or when lz4 has not been installed); when the buffers are compressed, there is an unavoidable moment where you have both the compressed and uncompressed copy in memory.