Create a service for deleting pieces of storage nodes.
Currently the DeletePieces method returns after a success threshold,
completion or a timeout.
The end goal is to return when reaching the success threshold and
leaving the remaining goroutines running after DeletePieces method
returns and add a life cycle to the service that it waits for them when
it closes.
This is the first commit for ticket:
https://storjlabs.atlassian.net/browse/V3-3476
Change-Id: If740bbf57c741f880449980b8176b036dd956c7b
