bootstrap: Only invoke config_callback directly once

Afterwards, use an async handle

Change-Id: Ib18a804b48d934c0e27009a84e6ffe570c4ced32
Reviewed-on: http://review.couchbase.org/32548
Reviewed-by: Brett Lawson <brett19@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>