Add support of dedicated backends

[knizhnik]

Problem

We are now going to provide connection pooling by default:

https://neondb.slack.com/archives/C04DGM6SMTM/p1737726792110159

The problem is that connections pooler in transactions or statement mode doesn't preserve session semantic.
It means that Postgres applications using

• GUCs
• temporary tables
• advisory locks
• sequence access functions (currval, nextval)
  will not work.

Summary of changes

We can still provide connection pooling by default, but enforce dedicated backends at Postgres level. In pgbouncer we are using transaction pooler mode. It means that pgbouncer will not reschedule backend to some other session until ned of transaction. End of transaction is determined by pgbouncer by backend status in ReadyForQuery message ('T' - transaction, 'I' - idle). We can add some checks to Postgres (I already done in in my pooler and it affects only very few places in Postgres core) which detects if connection need to preserve session semantic.
In this case we will always return 'T' status in ReadyForQuery message, preventing pgbouncer from reusing our backend.

See neondatabase/postgres#571

[github-actions]

7403 tests run: 7016 passed, 0 failed, 387 skipped (full report)

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Flaky tests (2)

Postgres 17

• test_metrics_normal_work: release-x86-64-without-lfc, debug-x86-64-without-lfc

Code coverage* (full report)

• functions: 33.5% (8498 of 25347 functions)
• lines: 49.3% (71497 of 144978 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
2ce2fac at 2025-01-27T07:03:41.115Z :recycle:}