store: use a larger buffer for eventChan

The event channel for watches can drop events too easily in the case of 
streaming watches. This increases the buffer to a more useful level.


Reference:
- https://github.com/coreos/etcd/blob/master/store/watcher.go#L69
- https://github.com/coreos/etcd/issues/1074

This is a suboptimal solution, but an improvement in anticipation of the store being reworked much more substantially.