Those two fields define the way sstable is stored as collection of
on-disk files. First step towards making the storage access abstract is
in moving the paths onto filesystem_storage embedded class.
Both are made public for now, the rest of the code is patched to access
them via _storage.<smth>. The rest of the set moves parts of sstable::
methods into the filesystem_storage, then marks the paths private.
Signed-off-by: Pavel Emelyanov <xemul@scylladb.com>