planner: support a command like 'load stats {table}' to enforce the planner to load stats into memory

## Enhancement
Support a command like 'load stats {table}' to enforce the planner to load stats into memory;
Two scenarios where this command can help:
1. add these commands into restarting scripts to let the planner can load some key tables' stats into memory when starting;
2. don't need to execute some queries to trigger the stats loading when testing.