VoteRequestor and FreshnessChecker need safe destruction

The VoteRequester and FreshnessChecker can be destroyed with callbacks still pending on their ScatterGatherRunner. Since the callbacks access the ScatterGatherRunner::Algorithm owned by the VoteRequestor/FreshnesssChecker, this can result in various bad behavior (crashes or memory corruption leading to just about anything). The ScatterGatherRunner::Algorithm needs to live until all callbacks are complete.

This would probably make sense to combine with SERVER-28389