persister: refactor to not be a global

[jskelcy]

With this PR a race condition has been in the brokertest package.

Johnny's comment on the issue:

But, there's a race here -- multiple NewBroker calls may be made by a single test, exercising having multiple discrete brokers coordinating through shared Etcd. One NewBroker can clobber the allocation action / newReplica call of another. I think the reason this isn't detected by the build now is that CreateJournals, used in tests, synchronously waits for allocation actions to complete before returning.

I'm thinking the proper fix is to have Persister not be a shared global - eg have broker.NewService & broker.newReplica take a SpoolComplete() interface argument, where Service plumbs the instance through to created replicas.

While this is not an issue currently the race should be resolved.