Add -work flag to 'bolt bench'.

### Overview 

This commit adds a 'work' flag to the bolt bench utility so that databases generated by the bench CLI can be saved for analysis.


### Example

```sh
$ bolt bench -work
work: /var/folders/p3/nqt2bgp17fn7_t9hr6tr_1m00000gn/T/bolt-bench-624397481
# Write	1.917837341s	(1.917837ms/op)	(521 op/sec)
# Read	1.000024224s	(32ns/op)	(31250000 op/sec)

$ bolt check /var/folders/p3/nqt2bgp17fn7_t9hr6tr_1m00000gn/T/bolt-bench-624397481
OK
```