Pass regressions tests in compute+pageserver setup

[kelvich]

That's is covering ticket for all the problems that will arise out of running pg_regress among with pageserver. Goal here is to get all redo edge cases / problems early on.

Some support routines to make testing easier:

> cd contrib/zenith_store
> make start # will start compute+pageserver and target them to each other using PostgresNode.pm
> make run-regress # starts pg_regress on compute

This task may done in a few steps:

1. Fix all pageserver crashes, ignoring test diffs
2. Fix test diffs itself, add proper synchronization of pageserver page version with latest evicted or written LSN.
3. When pg_regress passes it would make sense to write loop over all collected pages at pageserver and restore them all -- sicne there is possibility that not all pages were asked by processing node. This code would also make be helpful for snapshot export from pageserver.