Minimal implementation of the ranged (=threaded) segment loop
service, to improve performance over the existing loop.
Has tests with a an inmemory segment database
and example observer.
Does not have yet: database link, observer duration tracking,
suspicious processed ratio guard, rate limiting, minimum execution
interval per observer, etc.
Part of https://github.com/storj/storj/issues/5223
Change-Id: I08ffb392c3539e380f4e7b4f1afd56c4c394668d
