Fix UBRing macOS timer handling

[YChange01]

Summary

• allow macOS pthread spinlock shim to accept PTHREAD_PROCESS_PRIVATE
• fix UBRing timer cleanup and kqueue timer behavior on macOS
• isolate ubring_performance channels with connection_group and include protobuf headers for CMake builds

Test Plan

• cmake --build build --target brpc-static -j 8
• cmake --build example/ubring_performance/build -j 8
• build/test/test_butil --gtest_filter=ScopedLockTest.spinlock_process_private
• THREAD_NUM=4 ATTACHMENT_SIZE=1 TEST_SECONDS=5 PORT=8007 ./start_ubring_client.sh