MB-767: disable stats during incremental index phase

Change-Id: I47aa7ceea907c9b66d333d625fb0e9db1ebec0a2
Reviewed-on: http://review.couchbase.org/24161
Reviewed-by: Pavel Paulau <pavel.paulau@gmail.com>
Tested-by: Pavel Paulau <pavel.paulau@gmail.com>