detail::directory_log::nextPath() implementation.
detail::directory_log::nextPath() bug fix.
detail::directory_log::listDir() implementation.
detail::directory_log::lastPath() implementation.
DirectoryLog* refactoring.

DirectoryLog* -> directory_log::Log*
detail::directory_log -> directory_log::detail
Partial directory_log::Log{Reader,Writer} implementation.
directory_log::LogReader bug fix.