test_filestore_idempotent_sequence: put txn object in each collection

test_filestore_idempotent_sequence: put txn object in each collection

The transactions are idependent in each collection/sequencer, so we
can't record to a single txn object with racing transactions.  Fix
it by doing one in each collection, and when reading the latest op,
use the highest txn value we see.

Signed-off-by: Sage Weil <sage@redhat.com>