New snapshotting implementation

[donggyukim]

• Generate more understandable block rams(read-first mode)
  • Reference: http://www.xilinx.com/support/documentation/sw_manuals/xilinx2016_2/ug901-vivado-synthesis.pdf
• Reduce snapshotting overhead
• Generate null tokens during target reset

[donggyukim]

Validated with FPGA