[HBASE-20003] WALLess HBase on Persistent Memory - ASF JIRA

This JIRA aims to make use of persistent memory (pmem) technologies in HBase. One such usage is to make the Memstore to reside on pmem. Making a persistent memstore would remove the need for WAL and paves way for a WALLess HBase. The existing region replica feature could be used here and ensure the data written to memstores are synchronously replicated to the replicas and ensure strong consistency