Remove unnecessary Blob constructor

Blob has two very similar constructors, differing only if the Blob is
initially populated or not. Unify these two constructors to simplify
maintenance.

Change-Id: I1b77502cc178d0a116321fd32d5e0657a2fd9e09
Reviewed-on: http://review.couchbase.org/42776
Reviewed-by: abhinav dangeti <abhinav@couchbase.com>
Tested-by: abhinav dangeti <abhinav@couchbase.com>