Make MAX_GROUPING_TOKENS can be set outside of grouping.py

MAX_GROUPING_TOKENS helps on DDOS, but there is situation that I just want to run it offline for test purpose, which needs more than 10000. 

It is better that we can set MAX_GROUPING_TOKENS in our own module, like what we set the recursion limit by using `sys` like 
```
sys.setrecursionlimit(...)
```