Changes

bug fix and optimizations in NPC skins
fixed client sometimes disconnects for: Internal Exception:
io.netty.handler.codec.DecoderException: java.io.IOException: Packet
0/56 (ki) was larger than I expected, found 917 bytes extra whilst
reading packet 56; changes in Skin#setNPCTexture
Allow retrieving Skin instance by skin name instead of entity so it can
be retrieved without a spawned entity instance. Changes in Skin and
SkinPacketTracker.
add SkinPacketTracker#onSpawnNPC
remove redundant scheduled skin update tasks in CitizensNPC and
EventListen; provide no benefit
reset current SkinUpdateTracker instance if exists instead of creating
a new one for all players in EventListen#onCitizensReload
reset rotation count in EventListen when player teleports/changes
world/etc
reset update trackers for players nearby NPC's that spawn in case they
are not looking at the NPC.
 (commit: 80ee5f1)