Method process_batch

The thread take batches from the queue and apples them into memtable and WAL. Steps: 1. Grouping the batches in the queue into a big enough batch 2. Make sure there is enough space in the memtable. This might trigger a minor compaction or even several major compaction. 3. Write into WAL (.log file) 4. Write into Memtable 5. Update sequence of version set

(&self)