Modify init_delete_dir to be async

In the case when .delete directory contains lots of files.
The deletion of the directory leads to timeout in the supervisor.
Which in it's turn kills the node.
So we rename original directory so couchdb cannot find it.
Then we remove files from renamed directory asynchronously.

COUCHDB-2524
Btw, there is no need to provide magic "foo" word here. Just let Dir trail with slash as directory paths have to be and it will works without magic.