Projector fixes.

MutationTopicRequest(), RestartVbuckets(), AddBuckets():
  - TopicResponse will always contain current set of activeTimestamps and
    rollbackTimestamps from projector, even if the error field is non-empty.
  - vbuckets are ignored in requestTimestamps if there is an outstanding
    vbucket-request
  - ErrorTopicExist is removed from MutationTopicRequest() and AddBuckets().
  - Fixed inline comments for AddBuckets().
RestartTimestamps():
  - made idempotent, previously for ErrorFeeder error application should retry
    with AddBucket() was required.

Change-Id: Ibab2917d964e290e383137565c632a23397bc39d
Reviewed-on: http://review.couchbase.org/43240
Reviewed-by: Deepkaran Salooja <deepkaran.couchbase@gmail.com>
Reviewed-by: Pratap Chakravarthy <prataprc@couchbase.com>
Tested-by: Pratap Chakravarthy <prataprc@couchbase.com>