adapter: Optimize COPY FROM STDIN, parallelize it, use constant memory

[def-]

Tried with 100 million rows in https://github.com/def-/ClickBench/tree/pr-materialize/materialize. Before this PR it took 101 min, would go OoM if the files were not split, and have query errors because of results being too large. With this PR the 100 million row ingestion runs in 4:38 min on my dev server (8 cores), should scale pretty linearly with the number of cores. For reference COPY WITH FREEZE in PostgreSQL takes 20 min.

Example test run of the new benchmark in CI with 10 million rows: https://buildkite.com/materialize/release-qualification/builds/1089#019c68b4-4b3f-424f-b3c4-05518e95f1a0

NAME                                | TYPE            |      THIS       |      OTHER      |  UNIT  | THRESHOLD  |  Regression?  | 'THIS' is
--------------------------------------------------------------------------------------------------------------------------------------------------------
CopyFromStdin                       | wallclock       |           3.599 |         101.877 |   s    |    10%     |      no       | better: 28.3 times faster
CopyFromStdin                       | memory_mz       |        1310.349 |        1197.815 |   MB   |    20%     |      no       | worse:   9.4% more
CopyFromStdin                       | memory_clusterd |          28.458 |          28.534 |   MB   |    50%     |      no       | better:  0.3% less

Run in environmentd spec sheet doesn't scale well in Cloud (should be investigated!), but scales well locally:

cores    local    cloud
    1  178.20s  378.87s
    2   89.73s  170.18s
    4   44.79s  144.72s
    8   25.05s  158.65s
   16   20.72s  119.59s
   32      N/A  117.71s

Fixes: https://github.com/MaterializeInc/database-issues/issues/7674

Motivation:
COPY FROM STDIN has been slow for workload replay and testing with large amounts of data in general, also been annoying in https://github.com/MaterializeInc/database-issues/issues/7674 and recently https://materializeinc.slack.com/archives/C08A62E0751/p1770835109967349

[github-actions]

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