Memory configuration settings should be verified at startup

[cheddar]

There are a number of memory configuration settings that need to be set properly in order for the system to work. -XX:MaxDirectSystemMemory and druid.computation.buffer.size and druid.processing.numThreads are the ones that made me write this issue

The initialization process should check that it believes the settings will allow proper operation of the node and fail fast if this appear to be misconfigured.