Feature/issue10, add design doc for session expiration handling

[brucewu-fly]

new file:   doc/SESSION_EXPIRATION.md

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• InfluxQL Spec updated
• Provide example syntax
• Update man page when modifying a command
• Config changes: update sample config (etc/config.sample.toml), server NewDemoConfig method, and Diagnostics methods reporting config settings, if necessary
• InfluxData Documentation: issue filed or pull request submitted <link to issue or pull request>

[brucewu-fly]

https://github.com/chenziliang/influxdb/blob/feature/issue10-session-expiration-handling-design/doc/SESSION_EXPIRATION.md

[chenziliang]

1. Could you please rename SESSION_EXPIRATION.md to MASTER_SESSION_EXPIRATION.md since only the master handles the connection to etcd in phase 1 ?
2. Session expiration and auto connect is a builtin feature in etcd ? So please help investigate. what we will need do is just subscribe the session expiration event and reject all coming requests ?

[brucewu-fly]

@chenziliang Please take a look at these two files:
https://github.com/chenziliang/influxdb/blob/feature/issue10-session-expiration-handling-design/poc/etcd_client_auto_connect/auto_connect.go
https://github.com/chenziliang/influxdb/blob/feature/issue10-session-expiration-handling-design/poc/etcd_client_session_expiration/session_expiration.go

[chenziliang]

@brucewu-fly How do we know the session has expired and then reconnected from the code ? How to handle this session time out by using a callback or subscribe a timeout message ?

[brucewu-fly]

@chenziliang , added leaderelection example.
https://github.com/chenziliang/influxdb/tree/feature/issue10-session-expiration-handling-design/poc/leaderelection
https://github.com/chenziliang/influxdb/tree/feature/issue10-session-expiration-handling-design/poc/leaderelection_example