Get MDB txn before acquiring a CSN

Since MDB writes are fully serialized in txn_begin, we can use CSNs as
commit sequence numbers, not just change sequence numbers. This will
prevent most instances of out-of-order writes, so syncprov will almost
never need to transmit a change without a cookieCSN. (Out-of-order is
still possible, if the write was received via syncrepl.)