Add a new shared memory allocation strategy: fixed mode. In this mode, the master...

Add a new shared memory allocation strategy: fixed mode.  In this mode, the master core first allocates the 'local' shared memory chunks to each core.  When there are no more 'local' shared memory chunks for a core, the master core tries to allocate shared memory chunks that are belonging to the core's neighbour cores.  The priority of the neighbour cores are left-right-up-below.  If the master core failed again, it enables GC.