For the Reindex command, use a new threadpool defaulting to the number
of available processors, under the assumption that the bulk of
indexing time is in CPU-intensive work like tree diffing. Modify
ChangeIndexer to return ListenableFutures so we can bound the size of
the work queue.
For the server, default to using the default work queue, but make this
configurable as index.threads to use a dedicated threadpool, with
non-positive values indicating the default.
Because we now have more non-Lucene-specific Guice logic, split that
out into its own module.
Change-Id: I55181e556a2d43b81c9032f53b74b690342ab62b
c64399fe93