Track duration of all segment loop observers. Factor out functions to
reduce size.
Still need to send the measurements out via monkit.
Part of https://github.com/storj/storj/issues/5223
Change-Id: Iae0260e250f8ea33affed95c6592a1f42df384eb
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
