Create ByteBuffer backed Cell

As part of HBASE-12224 and HBASE-12282 we wanted a Cell that is backed by BB. Changing the core Cell impl would not be needed as it is used in server only. So we will create a BB backed Cell and use it in the Server side read path. This JIRA just creates an interface that extends Cell and adds the needed API.
The getTimeStamp and getTypebyte() can still refer to the original Cell API only. The getXXxOffset() and getXXXLength() can also refer to the original Cell only.