Add groupBy cleaning function, improve group by perf better than 4.x.
Add groupBy cleaning function, improve group by perf better than 4.x.

Introduce synchronised and unsafe state holders.

grainier/siddhi,gokul/siddhi,dilini-muthumala/siddhi,suhothayan/siddhi,gokul/siddhi,wso2/siddhi,tishan89/siddhi,ramindu90/siddhi,dilini-muthumala/siddhi,tishan89/siddhi,wso2/siddhi,mohanvive/siddhi,mohanvive/siddhi,grainier/siddhi,suhothayan/siddhi,ramindu90/siddhi