MB-10057: don't keep checkpoint status error message

Because it's not used anywhere

Change-Id: I840885e3e2152d05fdf23adef5c7ecc85a936024
Reviewed-on: http://review.couchbase.org/34032
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
MB-10057: killed unused source_seq field

Change-Id: I6ac2110f3f91634cc51f7bffe0e4dd24332c8d34
Reviewed-on: http://review.couchbase.org/34033
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
MB-10057: removed useless misc:flush(checkpoint) calls

Because checkpoint is not plain message but a gen_server:cast

Change-Id: Iba726f68f6bcb0eede22df820be8dd22ca0aa88f
Reviewed-on: http://review.couchbase.org/34034
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>