[pixels-core]: implement local file reader and writer.

[bianhq]

For some query engines like Raw, reading and writing Pixels file in local file system is needed.

[bianhq]

To support local storage, we need to generate the file (block) id by ourselves.
To do this, we upgrade etcd and use etcd to generate the id.

[bianhq]

We have implemented auto-increment id and the local storage.
By assigning a global unique id to each file and managing the mappings from 'constant_prefix:local_path:host_name' to file id, the local storage on each node can be seen as a simple distributed file system without replication. Thus pixels cache also can be used for local storage.