v2.0.0

- fully support and document DNS discovery using flag `--discovery-srv`
- additional tests for all packages
- fix the message flood from leader to follower when follower is unhealthy
- support scaling from two member clusters to single member clusters (not a recommended configuration)
- remove out of date documentation about the 0.4 releases
- use tls information internally when finding cluster membership
- WAL migration starts the term at 1 instead of 0
- add debug tool etcd-dump-logs
- update all documents to fit the v2.0 changes
- add `-heartbeat-interval` and `-election-timeout` flags

Thank you to: 周春寅, efrecon, Yicheng Qin, Xiang Li, Shota Fukumori (sora_h),
Rob Szumski, Peter Suschlik, Peter Lemenkov, Matthew Williams, Kelsey
Hightower, Jörg Thalheim, Jurriaan Mous, Jonathan Boulle, Brian Waldon, Brandon
Philips, Ben Darnell, Barak Michener, Andrew Bonventre, Alban Crequy