PUBDEV-8095: Add a warning if user tries to run XGBoost with not enough free memory

[tomasfryda]

https://h2oai.atlassian.net/browse/PUBDEV-8095

This PR consists of two parts:

• Expose off_heap_memory_ratio in h2o.init; off_heap_memory_ratio was introduced in https://github.com/h2oai/h2o-3/commit/d291cddc0bebf64c3eeed2e38ca019a255700352
• Add a default value to off_heap_memory_ratio in the h2o.init

This way the warning is generated only when the user started h2o using h2o.init or if somebody started the cluster and set the off_heap_memory_ratio to some non-zero value.