This patch moves the exfile.cc implementation to a C++ class with an
abstract interface of a file. The implementation of exfile.cc, renamed
to extents_file.cc, now uses C++ STL classes and has unittests to test
its core functionality.

Bug: 24478450
Test: Unittests added. make all test -j5 && ./unittests

Change-Id: I8d8f07150ad2ea465c55b5178ca9fbab49185eea