Separate init and runtime timeout budgets, add per-request timeouts

[moguchev]

Related Issue: #1919

Splits previously conflated timeout budgets so a low user-supplied Timeout no longer breaks session establishment, and adds dedicated client-side budgets for system queries and per-request overrides. Also fixes a potential controlConn reconnect deadlock.

• ClusterConfig.ReadTimeout: socket-level read deadline, defaults to 11s, falls back to ConnectTimeout if unset. Decoupled from Timeout, which is now purely a per-request client-side budget.
• ClusterConfig.Metadata.SystemRequestTimeout (default 60s): per-request budget for system/system_schema queries and other runtime control-conn frames. Applied via Conn.querySystem, controlConn.query and controlConn.writeFrame.
• Query/Batch.WithRequestTimeout / SetRequestTimeout / GetRequestTimeout: per-request override of ClusterConfig.Timeout, enforced via callReq.timer in Conn.exec independently of the socket deadline. 0 disables the client-side timer.
• Conn.finalizeConnection: switches an initialized connection from ConnectTimeout-bound init mode to operational ReadTimeout / WriteTimeout / Metadata.SystemRequestTimeout. Called by hostConnPool.connect, controlConn.connect and controlConn.attemptReconnectToAnyOfHosts. Until then, all socket reads/writes and internal init-phase requests (STARTUP/AUTH/OPTIONS/system.local/USE keyspace) are pinned to ConnectTimeout. Fixes "no connections were made when creating the session" when Timeout is low.
• controlConn.HandleError now launches reconnect in a goroutine to avoid a potential deadlock during a connection storm where attemptReconnect holds the controlConn mutex while doing DNS resolution / TCP dial.
• connReader.timeout, deadlineContextWriter.timeout and writeCoalescer.timeout are now atomics to allow finalizeConnection to update them concurrently with reader/writer goroutines.
• Regression tests: TestNewConnectWithLowTimeout, TestInitSchemaQueryRespectsConnectTimeout, TestControlConnQueryRespectsSystemRequestTimeout, TestQueryRequestTimeout, TestControlConn_HandleError_LaunchesReconnectInGoroutine.
• README: new "Connection Timeouts" section documenting the layers and picking guidance.

[joao-r-reis]

We want to prioritize a fix for a quick patch release so we need a PR that focuses on removing the connection read deadline and therefore making cfg.Timeout apply to just the request timeout (callReq timer).

After that we can work on a more comprehensive improvement of the existing timeout configuration fields such as this PR.

cc @worryg0d

[moguchev]

We want to prioritize a fix for a quick patch release so we need a PR that focuses on removing the connection read deadline and therefore making cfg.Timeout apply to just the request timeout (callReq timer).

After that we can work on a more comprehensive improvement of the existing timeout configuration fields such as this PR.

Of course, this PR is more for inspiration for the future works with timeouts.