First Iteration of size configurable GPU buffer for faster checkpoint capture

[Jaiaid]

Modification:

• A slot-based GPU buffer and host staging area where each slots are of 64 MiB size and the size of allocated memory is controllable by two optional config variables
  • gpu_buffer_slots: controls the number of slots in the GPU (default 8)
  • host_stage_slots: control number of slots in host, pinned if pinning is available (default 16)
• transfer engine update with two-thread transfer worker to accelerate capture when checkpoint size is small enough or GPU/host buffer is large enough
• GPU device is set to rank in round-robin manner

Correctness is checked by:

• checking if checkpoint restore works
• MPI run works
• performance values match with the system