pageserver: generate initdb on shard 0 only

[VladLazar]

Each shard runs its own initdb. Results need to be consistent for things to work.

More specifically:

• initdb needs to produce the same results
• The last record produced by initdb needs to not be shard specific. Otherwise we end up with inconsistent last_record_lsn across shards.

We should re-use the initdb generated by shard 0 on other shards.
Two implementation paths:

1. Shard 0 uploads initdb to s3, other shards poll S3
2. Shard 0 is created first, storcon sets existing_initdb_timeline_id in the timeline creation request for other shards which will prompt them to download instead of creating it inline.

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There's bolder ways in which we could change initdb generation (do it on compute), but they're not in scope here.