Upgrade to grpc_connection_pool 0.1.5 with improved connection management.

Highlights:
- Updated grpc_connection_pool: 0.1.3 → 0.1.5
- New DynamicSupervisor-based pool architecture
- Round-robin channel distribution for better performance
- Improved reliability with exponential backoff
- All tests passing (22/22)

No breaking changes to public API.