Increasing Mem when execute syn_scan task

[holiday-404]

When execute verifyDb(syn scan task),mem would incline strangely

On server
./db_stress/db_stress --n_partitions=100 --max_key=2000 --ops_per_partition=10 --keys_per_batch=300 --active_width=1000 --num_readers_per_partition=25 --max_verify_ops_per_write=50 --value_sz_mode=2 --syn_scan=true --db_path="mnt/ssd/eloq/stress_test"

[holiday-404]

I'm currently in the process of getting familiar with the codebase, so I haven't started working on this issue yet