Checkpoint progress for docs with _design prefix and too large bodies

If the last document in the sequence tree of a database file had its ID
with the "_design/" prefix, we would completley ignore it and skip
checkpointing. This was a problem if no other partition database files
had any updates and this document was associated with the last sequence
of the database - in this case each indexer run would process that sequence
number over and over, which would block forever stale=false queries - unless
a new document was added after to that database, or an existing one updated
or deleted. So just treat a document with a _design/ prefix in its ID as
any other document, as it shouldn't have any special meaning in regular
databases.

Similarly, if a document with a too large body is skipped, ensure that we
don't end in the same loop condition as above, by ensuring its sequence
number is checkpointed.

This was diagnosed in CBSE-893.

Change-Id: I7390db0329b00b137a0d9f1ff8e29971a58af143
Reviewed-on: http://review.couchbase.org/31341
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Sarath Lakshman <sarathlakshman@slynux.com>
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>