Adds block-based anti-entropy for the attribute stores and hooks into the existing `IndexSyncer` to regulate frequency of syncs.
This commit adds a `Closing` channel to the `IndexSyncer` and the `FragmentSyncer` that is periodically checked during execution. If the channel is closed then an in-progress sync is immediately stopped.
This commit refactors most of the code in `cmd/pilosa` to `pilosa.Server` so that it can be reused in long running cluster testing.
