Add key prefix matching to KEYS command (#237)

Related to #234 and !236.

This is the implementation that was requested in the original issue. I updated KEYS command to be redis-valid and implemented prefix search. There is also a rather interesting test, I could you use some feedback here.

I noticed that it might not be possible to reduce the complexity of the KEYS command. Because even if you use Scan, you will have to store the counter of all found keys before you do WriteBulk of the actual keys.

@prologic here is what you probably had in mind:

```
s.db.Scan([]byte(prefix), func(key []byte) error {
	conn.WriteBulk(key)
	return nil
})
```

But there is no way to call `conn.WriteArray(n)` with the number of keys until you iterate through all of them, hence the second loop over found keys.

Co-authored-by: Ivan Elfimov <ielfimov@gmail.com>
Co-authored-by: James Mills <james@mills.io>
Reviewed-on: https://git.mills.io/prologic/bitcask/pulls/237
Reviewed-by: James Mills <james@mills.io>
Co-authored-by: biozz <biozz@noreply@mills.io>
Co-committed-by: biozz <biozz@noreply@mills.io>

AUTHORS

@@ -17,3 +17,4 @@ Yash Chandra <yashschandra@gmail.com>
 Yury Fedorov orlangure
 o2gy84 <o2gy84@gmail.com>
 garsue <labs.garsue@gmail.com>
+biozz <ielfimov@gmail.com>