Use Processing-Time Clock in Window Assigners/Triggers

This is part of the effort to improve Window Triggers in Flink. We should replace all usage of System.currentTimeMillis() by a provided clock implementation. This way we can use a deterministic testing clock to verify the behavior of processing-time windowing components in unit tests.

This requires the following changes:

• Change StreamTask to have a Clock (name is WIP). By default this clock will use System.currentTimeMillis(). The clock must also provide an interface to register processing-time triggers, this is currently handled directly by StreamTask but must now also be handled by an external Clock
• Change API of WindowAssigner to take a context object that allows it to query the current processing time. This can be an abstract class AssignerContext with a single method long currentProcessingTime()
• Add a method long currentProcessingTime() in TriggerContext, change TriggerContext to use the clock methods provided by StreamTask and forwarded by WindowOperator
• Change processing-time triggers to use the new methods
• Change WindowOperator to support these changes