Improving leader efficiency via not processing learner's requests in commit processor

Currently, all the requests forwarded from learners will be processed like the locally received requests from leader's clients, which is non-trivial effort and not necessary to process those in CommitProcessor with session queue create/destroy

To improve the efficiency, we could skip processing those requests in leader's commit processor. Based on the benchmark, this optimization improved around 30% maximum write throughput for read/write mixed workload.