Local Gateway: Move shard state to be stored under each shard, and not globally under _state

[kimchy]

Currently, the shards state (which shards are active on a node) is stored in a global file in the (data) node. Move it to be stored on each shard location (under its own _state) so its collocated with the shard storage location, and, so we can optimize and only write shards that change their state and not everything each time.

The old shard state will automatically be upgraded to the new state location once its stated, with a backup created in case a revert to the old structure is required.