CBIDXT-14 Support for Initial Index Build

Initial Index Build is now supported from a separate Initial Stream.
And once the streams converge, Initial Stream is dropped and merged
into maintenance stream.

Change-Id: I04683befa614d4156ab65b565d2aac7bd311a390
Reviewed-on: http://review.couchbase.org/40868
Reviewed-by: Deepkaran Salooja <deepkaran.couchbase@gmail.com>
Tested-by: Deepkaran Salooja <deepkaran.couchbase@gmail.com>