Make Off heap Bucket Cache default ON

L2 cache can be used for data blocks. By default it is off now. After HBASE-11425 work, L2 off heap cache can equally perform with L1 on heap cache. On heavy loaded workload, this can even out perform L1 cache. Pls see recently published report by Alibaba. Also this work was backported by Rocketfuel and similar perf improvement report from them too.
Let us turn L2 off heap cache ON. As it is off heap, we can have much larger sized L2 BC. What should be the default size?