Batch delete action snapshots

[nhunzaker]

Microcosm master batch processes snapshots given a history range. We can do the same thing with removing snapshots. Basically, instead of:

_removeSnapshot(action: Action) {
  delete this.snapshots[action.id]
}

We could match the pattern for updating a snapshot range:

_removeSnapshotRange(source: Action, end: Action) {
  let focus = source
  while (focus && this.snapshots.hasOwnProperty(focus.id)) {
    delete this.snapshots[focus.id]
  }
}

There are two benefits:

1. History does not have to emit an action for every item it removes.
2. Microcosms can stop removing snapshots when they are out of snapshots

[nhunzaker]

Implementation details have changed here.