[FLINK-7655] [flip6] Set fencing token to null if not leader

[tillrohrmann]

What is the purpose of the change

This commit changes the fencing behaviour such that a component which is not the
leader will set its fencing token to null. This distinction allows to throw different
exceptions depending on whether it is a token mismatch or whether the receiver has
no fencing token set (== not being the leader).

Brief change log

• allow the fencing token to be null in FencedRpcEndpoint
• allow FencedAkkaInvocationHandler to send a null fencing token
• filter out messages if component is not a leader in FencedAkkaRpcActor
• Adapt Dispatcher, ResourceManager and JobMaster to set fencing token to null at start-up

Verifying this change

This change is already covered by existing tests, such as FencedRpcEndpointTest and AsyncCallsTest.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): (no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): (no)
• The serializers: (no)
• The runtime per-record code paths (performance sensitive): (no)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Yarn/Mesos, ZooKeeper: (no)

Documentation

• Does this pull request introduce a new feature? (no)
• If yes, how is the feature documented? (not applicable)

[tillrohrmann]

Travis passed. Merging this PR.