Add a new shared memory allocation strategy: mixed mode. Comparing to the fixed mode...

Add a new shared memory allocation strategy: mixed mode. Comparing to the fixed mode, there are following differences: 1) the neighbours cores are extended to the cores that a core can reach in no more than 2 flips; 2) introduced a threshold--if the master core can not allocate the neighbour cores' shared memory chunks, it checks current shared memory usage rate, if greater than the threshold it will enable gc, otherwise it tries to allocate the shared memory globally for the request core.