feat(single node): get a parameter to control memory usage of single node mode.

[KeXiangWang]

Currently, we haveRW_TOTAL_MEMORY_BYTES to control the memory usage of compute nodes. But in single node mode, it's confusing as it looks like a parameter to limit the whole RW's usage. Ideally, we should have another parameter to control the overall memory usage of single node mode. Or we can change the semantics of RW_TOTAL_MEMORY_BYTES in single node mode.