change DTM's memoryUsageChore default settings to enable memory threthold

DTM provide MemoryUsageChore to monitor the memory pressure in a RegionServer, when memory is beyond a defined threshold, it will give a warning or prevent new operations based on a flag.
By default, the threshold is now 100%, and the flag is just warning. So that means DTM coprocessor can use all RS memory and when it find memory pressure , just print warning, instead of control new transaction.
This will lead to un-predictable behavior for certain fat transaction.
It is better to change the default setting to enable protection.