fix: RELOAD/RECONNECT broke automatic role detection temporarily

[levkk]

fix #927

• fix: prevent concurrent RELOAD/RECONNECT
• fix: TOCTOU in fast/subsequent RELOAD commands pausing clients indefinitely (pretty unlikely but clearly possible race condition)
• fix: RELOAD/RECONNECT resets roles back to auto causing load balancer to send some writes to replicas

[codecov]

Codecov Report

❌ Patch coverage is 92.05298% with 12 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
pgdog/src/backend/pool/cluster.rs	90.90%	4 Missing ⚠️
pgdog/src/backend/pool/lb/mod.rs	92.50%	3 Missing ⚠️
pgdog-config/src/database.rs	92.30%	1 Missing ⚠️
pgdog/src/backend/auth/azure_workload_identity.rs	0.00%	1 Missing ⚠️
pgdog/src/backend/databases.rs	50.00%	1 Missing ⚠️
pgdog/src/backend/pool/shard/mod.rs	96.15%	1 Missing ⚠️
pgdog/src/backend/reload_notify.rs	91.66%	1 Missing ⚠️

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