only manually remove request if memory size change across container requests

we get report from our customer that when requesting memory like 3GB 3GB 1.5GB, sometimes the last 1.5GB request will not be received by RM. was able to repro this on one-box installation (it does not happen for 2GB 2GB 0.5GB though). 

This PR is to implement policy such that we do not remove the request if sequential requests are of different size, since this would not be necessary anyway (a different memory request coming in next will not be treated as the same as the previous one).