Allow users to control MaxDirectMemorySize

For people who use docker image and run flink in pods, currently, there is no way to update 

MaxDirectMemorySize

(Well one can create a custom version of taskmanager.sh)

As a result, it starts with a value of 8388607T . If the param taskmanager.memory.preallocate is set to false (default) the clean up will only occur when the MaxDirectMemorySize limit is hit and a gc full cycle kicks in. However with pods especially in kuberenete they will get killed because pods do not run at such a high value. (In our case we run 8GB per pod)

The fix would be to allow it be configurable via flink-conf. We can still have a default of 8388607T to avoid a breaking change.