Add --batch-size to 'bolt bench'

This commit adds a --batch-size CLI argument to the 'bolt bench' tool. This argument will insert into Bolt in smaller batches which is a more typical use case.

```sh
$ bolt bench --count 1000000 --batch-size 1000
# Write	2.731853335s	(2.731us/op)	(366166 op/sec)
# Read	1.032231032s	(35ns/op)	(28571428 op/sec)
```

/cc @snormore