MB-10117: Implementation of CMD_GET_KEYS api

Request:
- opcode:   CMD_GET_KEYS: 0xb8
- extlen:   4B
- ext:      | COUNT(4B) |

*if extlen = 0, then count defaults on 1000

Response
- Body:     |key1len(2B) |key1 |key2len(2B) |key2 |....
- Bodylen:  Total length of the buffer, sent in the body

Change-Id: I885f93e90114b62f63fa43822caac56cce15ac2a
Reviewed-on: http://review.couchbase.org/33417
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>