[BLOCKING] Upload tree building

[benbierens]

Found and captured in release tests + async profiler.
Found in bde9873 - fix(slotqueue): simplify slot queue workers (#1224)

Uploading a sufficiently large file (in this case 2GB) causes the node main thread to block for longer than the maximum allowed threshold.

Node log extract:

TRC 2025-05-16 07:54:50.720+00:00 Handling file upload                       topics="codex restapi" tid=1 count=39
INF 2025-05-16 07:54:50.720+00:00 Storing data                               topics="codex node" tid=1 count=40
TRC 2025-05-16 07:55:23.162+00:00 LPStreamChunker stream Eof                 topics="codex node" tid=1 exc="Stream EOF!" count=41
TRC 2025-05-16 07:55:27.642+00:00 Advertising                                topics="codex discoveryengine advertiser" tid=1 cid=zDv*N4aeQC count=42
TRC 2025-05-16 07:55:27.642+00:00 Advertising                                topics="codex discoveryengine advertiser" tid=1 cid=zDz*22US4e count=43
INF 2025-05-16 07:55:27.642+00:00 Stored data                                topics="codex node" tid=1 manifestCid=zDv*N4aeQC treeCid=zDz*22US4e blocks=32768 datasetSize=2147483648'NByte filename=none(string) mimetype="some(\"application/octet-stream\")" count=44
ERR 2025-05-16 07:55:27.642+00:00 Unhandled exception in async proc, aborting topics="codex" tid=1 msg="Execution time of future is too high (store[node.nim:391]) time: 2s896ms133us965ns threshold: 200ms" count=45

Warning: The same problem that's causing this issue might be to blame for #1227