Write stall cause memory usage peak

[flowbehappy]

Enhancement

Keep writing into an increasing PK(without shard_rows_bit) table with large throughput(over 15MB/s). The Avg_row_length is 1764.
Then try to restart tiflash:

1. The memory peak is most significant after restarting.
2. Write stall during the normal phase also causes high memory usage

[flowbehappy]

We need to change the default setting evict_cache_on_memory_ratio in tiflash proxy.

[hongyunyan]

It's same as #6399, thus close this issue.