[FFI] Add SharedLibrary class to track the usage of dynamic library

[yaoyaoding]

The ctypes.CDLL class does not provide a way to unload the loaded library. When a library is loaded, it will never
be unloaded until the program exits. However, when we tune an operator, we need to generate hundreds of kernels, and
each kernel will be compiled into a shared library. If we do not unload the shared library, we would load tens of
thousands of shared libraries, which will trigger memory error like:

cannot apply additional memory protection after relocation

To solve this problem, we need to unload the shared library after we use it. Thus, whenever we need to load a shared
library, we should use this module instead of the ctypes.CDLL class. The SharedLibrary class will keep track of the
loaded libraries, and unload them when no one references them.

The current implementation only supports *nix systems. Will add support for Windows when we plan to support Windows
in the project-level.