Use multiple threads in provider manager loop

[petar]

Currently the provider manager's loop is single-threaded, which causes an individual straggler request to the datastore to create a backup. Consider using multiple threads to alleviate the backup caused by few (a burst of) stragglers. Note that this solution would not help if all requests to the datastore are stragglers (which happens when the datastore itself is overlaoded).

[petar]

Duplicates #729