Clarify Tablet::Flush log messages
Clarify Tablet::Flush log messages

A lot of the log messages in DoCompactionOrFlush referred to a
"compaction" going on, when in fact the operation was a flush.  Change
the log messages to talk about a flush when that is what is happening.

Change-Id: I6fc33ab9ab6100fc06e2337dc16eb43c81fc272a
Reviewed-on: http://gerrit.ent.cloudera.com:8080/2354
Tested-by: jenkins
Reviewed-by: Michael Percy <e07a1fd797b473a9c849e618aa2ce06d8055760b@cloudera.com>

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