Closes#1332
The spawner delay feature has been broken in Essentials for as long as anyone can remember. The reasons for this are mentioned in the issue above.
This PR fixes this by changing the command to utilize new API for setting the minimum and maximum spawn delay on spawners. This API was added in 1.12.2, so all supported versions before that (1.8.8 thru 1.12.1) require NMS to function properly. I'm aware that Essentials avoids NMS for maintainability reasons, however that should not be of much concern here since all versions 1.12.2 and later are going to be using the Bukkit API. Hence, no NMS updates will be necessary.
Also let me know if you want the NMS code refactored somewhere else. I saw the net.ess3.nms packages, but I wasn't sure where this would fit into the organisation of that.
Tested on:
1.8.8, 1.9.4, 1.10.2, 1.11.2 (NMS)
1.12.2, 1.15.2 (Bukkit API)
