Call forceMerge(4) when an optimize() was called on the lucene index

git-svn-id: https://svn.apache.org/repos/asf/maven/indexer/branches/lucene-4.6@1575157 13f79535-47bb-0310-9956-ffa450edef68
git-svn-id: https://svn.apache.org/repos/asf/maven/indexer/branches/lucene-4.6@1575158 13f79535-47bb-0310-9956-ffa450edef68
Use single segment for 'optimize', make sure commit() is called afterwards.

git-svn-id: https://svn.apache.org/repos/asf/maven/indexer/branches/lucene-4.6@1575159 13f79535-47bb-0310-9956-ffa450edef68