KIJIMR-115. Improve AvroMapReader javadoc / api

- Clarifies JavaDoc.
- read() renamed to reload()
- Fixes a bug where previously calling read() wouldn't reflect deletions
- mMirror populated at creation. getMirror() removed
- Adds a unit test for reload() behavior
- Fixes a bug where some state changes would be reflected without calling reload()

issue: https://jira.kiji.org/browse/KIJIMR-115
review: https://review.kiji.org/r/552