Increase .delete_dir concurrency

[slowjazz]

Store.delete_dir() appears to run serially. Can we use something like concurrent_map to improve this operation?

[d-v-b]

the best method for speeding this up depends on the storage backend. It will look very different on object storage (where all the keys can be iterated over easily) vs a local file system (where you have to recursively list each directory).

[slowjazz]

got it. I'm mainly interested in having this for obstore at the moment, so i can close this if those changes look good.