Add batch-size option

[nrhorner]

Hi Derrick,

I am interested in using kraken2 in a real-time server/client application where clients stream data from a single sample to a a long running kraken 2 server. Input sequences are read in via stdin and streamed back to the client, without having to reload the databases between each client connection.

This MR adds a --batch-size option, which forces the use of reader1.LoadBatch(batch_size) instead of reader.LoadBlock() for non-paired-end reads. This makes it easier to keep track of how may samples have been processed by the server and speeds things up quite a bit.

I hope you're able to include it.

Thanks,

Neil