indexer: Handle DCP snapshot marker types correctly

The indexer should process any snapshot marker type with
on-disk (0x1) or in-memory (0x2) flags set.

Change-Id: I5b2449e4b5fad7ddea165eb057456815e249b619
Reviewed-on: http://review.couchbase.org/43584
Reviewed-by: Deepkaran Salooja <deepkaran.couchbase@gmail.com>
Reviewed-by: Sarath Lakshman <sarath@couchbase.com>
Tested-by: Sarath Lakshman <sarath@couchbase.com>