Remove unused methods from RadosFsFile

Those are createStripes and removeStripes.
Add new file write method with buffer copy option

This allows the user to delegate the responsibility of deleting the
given buffer to the file instance which can be convenient.
test: Write with buffer copy option in FileReadWrite
Add missing include for sys/stat.h
build: Remove FindGTest.cmake

This file is not needed because CMake already ships with it (at
least since 2.8).