Add explicit Item copy-constructor, reduce uses of value_t constructor

Further refactor and simplify Item construction:

- Add an explicit Item copy-constructor for use when copying items
  (e.g. MutationResponse).

- Change uses of Blob-passing constructor to just use the {char* data,
  size_t len} variant instead - in three use-cases the client just
  creates a Blob immediately before calling the constructor; might as
  well let the consturctor do the work.

This reduces the users of the Blob-passing constructor to one -
StoredValue::toItem(). This already has a value_t and so we do want to
make the new Item point to it, hence can't change this to use the
{data, len} variant.

Change-Id: I7c0ff575da70b24290c49d371e1aff14ef160d55
Reviewed-on: http://review.couchbase.org/42626
Reviewed-by: abhinav dangeti <abhinav@couchbase.com>
Tested-by: abhinav dangeti <abhinav@couchbase.com>
Remove unused Blob New() method

Code cleanup - no-one uses this method so delete it.

Change-Id: I10df43e052a96832fe9b4c882be6a19497392d38
Reviewed-on: http://review.couchbase.org/42775
Reviewed-by: Sundararaman Sridharan <sundar@couchbase.com>
Reviewed-by: abhinav dangeti <abhinav@couchbase.com>
Tested-by: abhinav dangeti <abhinav@couchbase.com>