Fix connection leak by request timers not cancelled in time

[BewareMyPower]

Fixes#554

Motivation

After a pending GetLastMessageId request is removed from pendingGetLastMessageIdRequests_ (e.g. in handleGetLastMessageIdResponse), the associated timer is not cancelled, so the callback, which captures the shared_from_this() of ClientConnection, still exists in the executor's internal queue. We should cancel the timer when it's removed.

Modifications

Add a PendingRequest abstraction to replace the manually written struct of the promise and timer, then cancel the timer in its destructor.

Add ClientTest::testConnectionNotReferredAfterClose to verify the change works that the close time won't be too long. Add testTimedOutPendingRequestsAreErasedFromConnectionMaps to verify the timeout logic is handled well in PendingRequest.

[Copilot]

Pull request overview

This PR addresses #554 by refactoring how ClientConnection tracks in-flight requests and their timeouts, aiming to avoid connection objects being kept alive by outstanding timer handlers.

Changes:

• Introduces a PendingRequest<T> helper that owns a timer + promise and uses weak_from_this() in the timer handler.
• Adds ExecutorService::createTimer() to construct an ASIO::steady_timer with an expiry.
• Migrates multiple ClientConnection pending-request maps (requests/lookups/stats/schema/etc.) to store PendingRequest instances and removes the old consumer-stats sweep timer logic.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 6 comments.

File	Description
lib/PendingRequest.h	New helper abstraction for request promise + timeout timer.
lib/ExecutorService.h	Adds a createTimer() convenience factory for steady_timer.
lib/ClientConnection.h	Replaces several pending-request structs/maps with PendingRequest-based maps (mostly unordered_map).
lib/ClientConnection.cc	Switches request creation/timeout wiring to PendingRequest, removes consumer-stats periodic timer cleanup.

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