[FLINK-2354] [runtime] Replace old StateHandleProvider by StateStorageHelper in ZooKeeperStateHandleStore

The old StateHandleProvider used in ZooKeeperStateHandleStore had to be replaced because the state backend implementation has changed. Since the new state backend could not be used anymore, a new StateStorageHelper interface has been created. The default implementation FileSystemStateStorageHelper stores the given state onto the specified file system and returns a FileSerializableStateHandle.

Various fixes due to rebasing.

Squashes:
5a88d5e [tests] Add HttpTestClient for testing HTTP responses
656d6d6 Split WebMonitor and LeaderRetrievalService start up
a7e8da8 Move generated /web files to src/main/resources

Add comment to webMonitorPort attribute and make line breaks more Scalaesque

Don't block on leader retrieval and only resolve associated job manager once

Make JobManagerRetriever independent of redirecting logic

This closes #1202.

Add README.md that outlines build instructions.