feat(flex): Configure the memory strategy with memory-level.

[luoxiaojian]

Passing parameter "--memory-level=n" to rt_server to configure memory allocation strategy.

• n = 0: memory allocation will be mmap to files in SHARED mode. Suitable for scenarios with severe memory constraints.
• n = 1: memory allocation will be mmap to files in PRIVATE mode. The modified parts in the storage cannot be swapped to disk; if the modified portions become too large, it may lead to a crash due to insufficient memory.
• n = 2: the storage of certain data will utilize hugepages, which can enhance the system's throughput. However, the portion using hugepages will be fixed in memory usage, and hugepages require static allocation.
• n = 3: the storage of all data will endeavor to make use of hugepages as much as possible.