Add a "metronome" for spreading service call load

Summary:
Ref T13244. See D20080. Rather than randomly jittering service calls, we can give each host a "metronome" that ticks every 60 seconds to get load to spread out after one cycle.

For example, web001 ticks (and makes a service call) when the second hand points at 0:17, web002 at 0:43, web003 at 0:04, etc.

For now I'm just planning to seed the metronomes randomly based on hostname, but we could conceivably give each host an assigned offset some day if we want perfectly smooth service call rates.

Test Plan: Ran unit tests.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13244

Differential Revision: https://secure.phabricator.com/D20087