MB-12633: Don't sleep right after scheduling a backfill manager task

Change-Id: I97dd4a91b036e4334f8338b6a6831ac959ac5773
Reviewed-on: http://review.couchbase.org/43103
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>