Increase default value for twill reserved memory mb
We've seen several issues where the YARN nodemanager will kill containers because they exceeded their physical memory allocation. The fix is to increase twill reserved memory, to reserve more memory for non-heap usage. I've seen things fail with 512mb, which is double the default 256mb.
I think we should increase the default value to 768.
Increased default twill reserved memory from 300mb to 768mb in order to prevent YARN from killing containers in standard cluster setups.