Support :nx, :xx, :ex, :px options for SET command

Based on:

  https://github.com/causes/mock_redis/pull/71

with minor tweaks.

Signed-off-by: Greg Hurrell <greg@causes.com>
Change-Id: I6cc1fe8544a02e8ce673ef9fa6154c7a61ca6112
Reviewed-on: https://gerrit.causes.com/30771
Tested-by: jenkins <jenkins@causes.com>