completely async request proxying + jetty update

add docs

doc for inverted topN metric spec

Allow router to override selection based on config

Conflicts:
	services/src/main/java/io/druid/cli/RouterJettyServerInitializer.java

Fix race in RemoteTaskRunner that could lead to zombie tasks.

upgrade to jetty 9.2.1

Upgrade jetty version
- supports Java8
- Async I/O Proxying
- other jetty bug fixes

Add way to skip determine partitions for index task

AWSCredentialsProvider for s3-extentions

Add a way to skip determinePartitions for IndexTask by manually
specifying numShards.

some minor cleanups to ingest firehose

Conflicts:
	server/src/test/java/io/druid/realtime/firehose/CombiningFirehoseFactoryTest.java