Backport of d2d2d3907f to stable-2.12.
The NIOFSDirectory type does not handle being interrupted well, as an
interrupt delivered by an SSH connection closing will close the file
handles used to read the index.
Isolate the Lucene reader from the SSH threads by always running a
Lucene query on the interactive index thread pool. This may reduce
latency for user dashboards on a multi-core system as each query for
the different sections can now run on separate threads and return
results when ready.
Bug: Issue 4400
Change-Id: I9fd97298d000e2b32371b9e0c454f7db3ef867ab
19ef754888