feat: wait for primary to be detected

[levkk]

fix #927

[general]
read_write_strategy = "conservative_primary_required"

This will pause all queries until a primary is detected by lsn_check_interval. This will prevent this error:

 cannot execute INSERT in a read-only transaction

since PgDog won't allow any queries through until a primary is detected, so a write won't be sent to a replica.

This is not default behavior because primary databases are optional, i.e. pgdog can proxy a set of replicas only, so a primary will never be detected.

[codecov]

Codecov Report

❌ Patch coverage is 51.35135% with 18 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
pgdog/src/backend/pool/shard/mod.rs	33.33%	10 Missing ⚠️
pgdog/src/backend/pool/lb/mod.rs	0.00%	5 Missing ⚠️
pgdog/src/backend/pool/cluster.rs	71.42%	2 Missing ⚠️
pgdog/src/backend/pool/shard/monitor.rs	0.00%	1 Missing ⚠️

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[levkk]

#941