First step towards treating all memory as uint64_t instead of uint32_t.

Change RPCs and add encode/decode logic for past version.

Change variables and structures to uint64_t:
def_mem_per_cpu, max_mem_per_cpu, free_mem, real_memory, actual_real_mem,
job_mem_lim, step_mem_limit, pn_min_memory, job_mem.

Change MEM_PER_CPU flag to 64th bit instead of 32nd bit.

Note that additional work is required to address local variables.

Also need to add table conversion logic to MySQL, as mem_req column
needs to change type to 'bigint unsigned' from 'int unsigned'.