Lazily realize txes from tx-range

Hi there,

I noticed that calling `last` on line https://github.com/RallySoftware/datomic-replication/blob/9ca1cbc3e4ad335bb4bb0c5380123e4ade907e5d/src/datomic_replication/core.clj#L103 will fully realize `txs`. I've rewritten the function with `letfn` to only look at `:t` for the current transaction.