GDKinit: reduce GDK_mmap_minsize & GDK_mem_bigsize by number of threads
GDKinit: reduce GDK_mmap_minsize & GDK_mem_bigsize by number of threads

With T concurrent threads, we reduce GDK_mmap_minsize & GDK_mem_bigsize
by 1/T such that the combined size does not exceed physical memory.

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