Disable merging of Iteration Aux Tasks

This will become obsolete once we have buffer oriented execution.
Disable it for now because it is causing a bug: FLINK-1087
Fix DeltaIteration Bug with Immutable Types

The CompactingHashTable did not obey the new serialization contract that
allows for immutable objects. We must always use the return value of the
deserialization methods and cannot assume that the data was put into the
reuse object.
[FLINK-1086] Replace JCL with SLF4J and Log4j with LOGBack

- Excluded Kafka's transitive dependencies: jmxtools and jmxri
- Corrected encoder pattern in logback.xml
- Removed explicit logging access. Loggers are now configured by
  configuration files. Fixed Yarn issue with multiple logging
  bindings.

This closes #111.
[FLINK-1017] Add documentation for execution parallelism

This closes #114
Updated documentation figure: FlinkOnYarn.svg

This closes #110
[FLINK-629] getFieldNotNull added to Tuple and updated Aggregators and Comparators to use that where appropriate
[FLINK-629] Updated getFieldNotNull usage and added it to TupleSerializers

This closes #73
[FLINK-1063] Revert race introducing commit

This reverts "[FLINK-998] Close network connections when idle" [1] as
it introduced a race condition into the network stack, which might
result in a re-ordering of network envelopes.

[1] 52512636444902497e47ccbfb1cabaffb3e23343