CHANGELOG.md inconsistencies across branches

Current problems:
- [ ] [master has 1.5.5 changelog](https://github.com/elastic/logstash/blob/master/CHANGELOG.md): since master is on 3.0.0.dev this branch should no longer receive changelogs from 1.x or 2.x since 2.0.0 GA
- [x] [2.x has 1.5.5 changelog](https://github.com/elastic/logstash/blob/2.x/CHANGELOG.md): 2.x should only have 1.5 logs before 2.0.0-beta1
- [ ] [2.1 only has changelog up to 2.0.0-beta2](https://github.com/elastic/logstash/blob/2.1/CHANGELOG.md): it should have everything up to 2.0.0 GA

Compare the linked CHANGELOG.md files on master, 2.x, and 2.1, checking each branch against the version boundaries listed in the issue. Update the branch changelogs so master no longer includes 1.x or 2.x entries, 2.x stops 1.5 entries before 2.0.0-beta1, and 2.1 includes entries through 2.0.0 GA.

Written by the indexing model from the issue text.