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
Hmm very good point. Maybe I should use random suffix. And remove files in every directory with given prefix. I.e. ".delete.tmp.*"