Draft: Rewrite the pthread_rwlock-based isc_rwlock for better tracing

While using mutrace, the phtread-rwlock based isc_rwlock implementation would be all tracked in the rwlock.c unit losing all useful information as all rwlocks would be traced in a single place. Rewrite the pthread_rwlock based implementation with just macros, so we can use mutrace to properly track the rwlock contention without heavily patching mutrace to understand the libisc synchronization primitives.