Improvements to Indexer Recovery

- Fix a few issues with timekeeper in enable/disable flush during
merging the catchup stream
- also handle the case when stream has been merged partially (ie. for
a subset of buckets) and there is another failure
- batch support in removeindex in kv_sender
- stringify streamId, stream state, index state in logging

Change-Id: Iacd437ac298c385c7f13b4972f246e28f438c4e5
Reviewed-on: http://review.couchbase.org/42281
Reviewed-by: John Liang <johliang00@gmail.com>
Reviewed-by: Deepkaran Salooja <deepkaran.couchbase@gmail.com>
Tested-by: Deepkaran Salooja <deepkaran.couchbase@gmail.com>