RLQS: Improve timer lifetime management

[yanavlasov]

1. Refactored GlobalRateLimitClientImpl to inherit from std::enable_shared_from_this.
2. Updated all asynchronous callbacks in GlobalRateLimitClientImpl to capture a weak_ptr to 'this' and use the lock-and-check pattern.
3. Changed TlsStore to hold a std::shared_ptr and implement a custom deleter for std::shared_ptr to ensure main-thread cleanup, even when dropped by worker threads.
4. Updated LocalRateLimitClientImpl to hold a std::shared_ptr, anchoring the lifetime of the global client and its resources for the duration of the filter.
5. Updated unit tests to align with the new lifetime management model.

Risk Level: medium
Testing: unit tests
Docs Changes: n/a
Release Notes: n/a
Platform Specific Features: n/a

[yanavlasov]

@bsurber for review

[bsurber]

LGTM once you have the last bit of coverage