Made Rock::Rebuild an AsyncJob because it is.

Made Rock::Rebuild an AsyncJob because it is.

Increment StoreController::store_dirs_rebuilding early, when SwapDir is
created and before the disk db file is opened and the actual rebuild starts.
Otherwise, if one SwapDir finishes rebuild before others start,
storeRebuildComplete() will see StoreController::store_dirs_rebuilding equal
to one, and think the rebuild is over.

This was not a problem for cache_dirs using blocking I/O because they either
did not try to open some file at SwapDir::init() time or did so synchronously,
resulting in "immediate" StoreController::store_dirs_rebuilding increment from
Store init loop point of view.