This reverts commit 804f1d9c19.
The intention was to drain and close the executor before closing the
index but closing an index doesn't necessarily means that executor
should be closed. After online reindexing is completed, the old index
version is closed which closed the executor and make any subsequent
Lucene interaction fails.
Bug: Issue 4618
Change-Id: I6ec90eb73312008714aa790308e806a0134a124e