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## Describe your changes Do not pin current block in BlockCursor ## Issue ticket number and link See #3712 There are places (see get_reconstruct_data) in our code when thread is holding read layers lock and then try to read file and so lock page cache slot. So we have edge in dependency graph layers->page cache slot. At the same time (as Christian noticed) we can lock page cache slot in BlockCursor and then try obtain shard lock on layers. So there is backward edge in dependency graph page cache slot>layers which forms loop and may cause deadlock. There are three possible fixes of the problem: 1. Perform compaction under `layers` shared lock. See PR #3732. It fixes the problem but make it not possible to append any data to pageserver until compaction is completed. 2. Do not hold `layers` lock while accessing layers (not sure if it is possible to do because it definitely introduce some new race conditions). 3. Do not pin current pages in BockCursor (this PR). My experiments shows that this cache in BlockCursor is not so useful: the number of hits/misses for cursor cache on pgbench workload (-i -s 10/-c 10 -T 100/-c 10 -S -T 100): ``` hits: 163011 misses: 1023602 ``` So number of cache misses is 10x times larger. And results for read-only pgbench are mostly the same: ``` with cache: 14581 w/out cache: 14429 ``` ## Checklist before requesting a review - [ ] I have performed a self-review of my code. - [ ] If it is a core feature, I have added thorough tests. - [ ] Do we need to implement analytics? if so did you add the relevant metrics to the dashboard? - [ ] If this PR requires public announcement, mark it with /release-notes label and add several sentences in this section.