Support backpressure for sharding

[petuhovskiy]

Add shard_number to PageserverFeedback and parse it on the compute side. When compute receives a new ps_feedback, it calculates min LSNs among feedbacks from all shards, and uses those LSNs for backpressure.

Add test_sharding_backpressure to verify that backpressure slows down compute to wait for the slowest shard.

[github-actions]

2706 tests run: 2574 passed, 0 failed, 132 skipped (full report)

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Code coverage* (full report)

• functions: 28.3% (7127 of 25167 functions)
• lines: 46.8% (43716 of 93313 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
8031bd0 at 2024-03-18T21:14:21.993Z :recycle:}

[jcsp]

Nice change -- some comments inline. Let's figure out whether we're going to want ShardCount in PageserverFeedback for later safekeeper logic: if so, it probably makes sense to add it now while we're changing the format.

(I leave it to the postgres experts to review the C changes)

[arssher]

Approved, but let's fix graceful shutdown before merge.