Indexing service API and GUI improvements!

- New APIs: waitingTasks, completeTasks, task payload
- GUI for the above, and for task logs + status
TaskStorage: Return recently complete tasks in reverse chronological order
add better messaging and error handling
better messaging to console again
Configurability of recency threshold
Merge pull request #327 from metamx/index-service-api-gui

Indexing service API and GUI improvements!
prepare for next release
[maven-release-plugin] prepare release druid-0.6.34
[maven-release-plugin] prepare for next development iteration
fix broken task storage config and prepare for next release
[maven-release-plugin] prepare release druid-0.6.35
[maven-release-plugin] prepare for next development iteration
fix task storage config serde and prepare for next release
[maven-release-plugin] prepare release druid-0.6.36
[maven-release-plugin] prepare for next development iteration
fix typo
S3Utils: Fix retry predicate
ZkCoordinator: Make addSegment, addSegments idempotent
Coordinator: Stop databaseRuleManager in stopBeingLeader
Coordinator: Link service announcement to leadership