Release v0.3.1

This version starts to support kubernetes with undermoon-operator and fixes a data loss bug.

Broker Data Format Change

To upgrade the memory broker from 0.3.0 to 0.3.1,
we need to use the script

> python scripts/mem_store_v1_to_v2.py metadata new_metadata

to migrate the memory broker data.

Bug fixes compared to 0.3.0:

• Temporarily losing keys during migration:
  Fix temporarily losing keys during migration if there's no many keys to migrate

• May lose all the data of the same chunk after failover:
  Fix incorrect slots position after failover
  This could be fixed by balancing the masters back to the normal status.

Features:

Add config api for memory broker
Add apis to query epoch and broker config
Add auto scaling api
Support Kubernetes StatefulSet for memory broker
Fix Kubernetes mode
Support UMCTL READY for Kubernetes readiness
Add default_redirection_address for scaling down
Support inner retry for the server proxy
Remove recursive call in blocking module
Amend proxy readiness status
Use other crc16 hash function for key lock in migration
Fix ready status
Fix INVALID_NODE_NUMBER

Docs:

Add development guide
Add docs on broker config api
Add docs on operator. Add script to migrate v1 broker data

Others:

Support downloading undermoon image in the readme example
Rename overmoon to broker in chaostest