Move isFamilyEssential(byte[] name) out of Filter interface in 0.94

As Dave Latham pointed out here:
https://issues.apache.org/jira/browse/HBASE-5416?focusedCommentId=13584553&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-13584553

The addition of isFamilyEssential(byte[] name) in Filter interface in 0.94.5 broke backward compatibility.

Agreement in subsequent discussions on HBASE-5416 was reached: isFamilyEssential(byte[] name) should be lifted from Filter interface into FilterBase class.

This would allow 0.94.6 to be compatible with 0.94.1 to 0.94.4 releases.