Initailize tap stat "sent_from_vb" to 0 for all vbuckets

On some platforms sometimes the integer values may not be initailized
to 0 by default. So explicitly do it to the vector 'transmitted'
in the TapProducer contructor.

This fix the sporadic failure from est_vb_move test which depends on
the stat.

Change-Id: Iacb7942ecc0f2991ffa3e053476031e01d0ec239
Reviewed-on: http://review.couchbase.org/31637
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>