refactor(server): improve statement timeout and parameter handling

[tjhanley]

• Add statement timeout configuration at pool level (pools.<pool_name>.statement_timeout)
• Add logging for statement timeout configuration during pool initialization
• Improve sync_parameters method to handle statement timeout consistently
• Document statement timeout configuration in CONFIG.md
• Change string initialization from String::from() to String::new()
• Maintain cleanup state until connection check-in for proper connection pooling

The statement timeout setting leverages PostgreSQL's native statement_timeout parameter, allowing the database to handle query termination automatically. This ensures reliable timeout enforcement directly at the database level, rather than managing timeouts in the connection pooler. Pool-level settings take precedence over user-level settings for consistent timeout enforcement across all pool users.

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---

Connecting to the Postgres instance through PGCat connection pooler.

Minimal Config

[general]

host = "0.0.0.0"
port = 6433
admin_username = "pgcat"
admin_password = "pgcat"

[pools.pgml]
statement_timeout = 30000

[pools.pgml.users.0]
username = "postgres"
password = "postgres"
pool_size = 10
min_pool_size = 1
pool_mode = "transaction"

[pools.pgml.shards.0]
servers = [
  ["127.0.0.1", 28815, "primary"]
]
database = "postgres"

SELECT 
    state,
    current_setting('statement_timeout') AS statement_timeout
FROM pg_stat_activity
WHERE state = 'active';

state	statement_timeout
active	30s

---

Removing the option from the configuration yields this in the config.

2025-02-25T20:59:05.014649Z  INFO ThreadId(01) pgcat::config: [pool: pgcattest] Statement timeout: 0

SELECT 
    state,
    current_setting('statement_timeout') AS statement_timeout
FROM pg_stat_activity
WHERE state = 'active';

state	statement_timeout
active	0