Make Index Manager Optional

Till the Indexer/Index Manager integration is complete, making the
Index Manager optional to make things easier.

There is a flag enable_manager which can be set to true while
running Indexer to make it use Index Manager.
By default, enable_manager is set to false.

Change-Id: I250ccf9bc633ba2fa22cf5c28ccf15368ed94ce4
Reviewed-on: http://review.couchbase.org/42509
Reviewed-by: John Liang <johliang00@gmail.com>
Reviewed-by: Deepkaran Salooja <deepkaran.couchbase@gmail.com>
Tested-by: Deepkaran Salooja <deepkaran.couchbase@gmail.com>