Introduce crcutil library

This commits adds a possibility to use crcutil library in the LizardFS
code. This library is automatically downloaded when cmake is being run.
common: Use crcutil for CRC calculation

This commit changes the implementation of the CRC algorithm to
the one provided by crcutil library. The new code is rougly 3
times faster than the previous implementation and should result
in a noticeable decrease of CPU usage on some installations.
tests: Add GoogleTest framework

This commit provides capability to make unit-tests using GoogleTest framework.
Framework is downloaded automatically to source directory (external/gtest*)
during project configuration.