1. Add mutex when operating the memory pool.
2. Support CMake 3.19 and fix some CMake bugs. Note that CUDA_ROOT variable in CMake is modified as CUDA_TOOLKIT_ROOT. You can find this update in the README.
3. Fix some bugs on Ubuntu.

1. Add mutex when operating the memory pool.
2. Support CMake 3.19 and fix some CMake bugs. Note that CUDA_ROOT variable in CMake is modified as CUDA_TOOLKIT_ROOT. You can find this update in the README.
3. Fix some bugs on Ubuntu.

impove the implementation of the wating function. Now we replace rolling as a queue to check if the jobs are finished.