this preserves the existing behavior of `configure.py` in the CMake
generated `build.ninja`.
* configure.py: map 'release' to 'RelWithDebInfo'
* cmake: rename cmake/mode.Release.cmake to cmake/mode.RelWithDebInfo.cmake
* CMakeLists.txt: s/Release/RelWithDebInfo/
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>