Memory usage raise about 4 GB when there are stale snapshot

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

# build and patch tiflash with failpoint enabled release version
> cmake -DCMAKE_BUILD_TYPE=RELWITHDBGINFO -DENABLE_FAILPOINTS=ON -GNinja ..
# use br to load chbenchmark 1500 from internal minio service

# enable failpoint for holding the snapshot for a while
> LD_LIBRARY_PATH=. ./tiflash client --host ${tiflash_ip} --port ${tiflash_tcp_port} --query 'DBGInvoke enable_fail_point(force_slow_page_storage_snapshot_release)'
> LD_LIBRARY_PATH=. ./tiflash client --host ${tiflash_ip} --port ${tiflash_tcp_port} --query 'selraw count(*) from db_69.t_79'
471415523

https://github.com/pingcap/tiflash/blob/e3a44124f17482bab9039f1b60fc275fe5acfbf2/dbms/src/Storages/DeltaMerge/DeltaMergeStore.cpp#L1139-L1145

2. What did you expect to see? (Required)

After the snapshot created, the memory usage do no change significantly

3. What did you see instead (Required)

After the snapshot created, the memory usage raise about 4 GB, without any write operations.
After the snapshot released, the memory usage will drop to normal level.

http://172.16.5.81:21510/d/SVbh2xUWk/jayson-tiflash-summary?orgId=1&from=1654597201960&to=1654599606104

4. What is your TiFlash version? (Required)

https://github.com/JaySon-Huang/tiflash/commits/debug_ps base on e3a44124f17482bab9039f1b60fc275fe5acfbf2