Storage Engine: optimize wal snapshot or flush threshold

[caozj1011]

Description

Adjust the memTable snapshot or flush threshold. Change from iot_consensus_throttle_threshold_in_byte * 0.8 to iot_consensus_throttle_threshold_in_byte * 0.6. when wal disk usage small than iot_consensus_throttle_threshold_in_byte * 0.6, will snapshot or flush oldest memTable util greater than it.