Properly copy collection references in ItemMeta.clone(). Fixes BUKKIT-3913

When cloning an item, all references are copied to the new item. For
collections, this makes internal changes become visible in both the old and
new items.

In CraftMetaItem, clone was not making copies of the appropriate collections
and has been fixed for non-null values.

In CraftMetaEnchantedBook and CraftMetaPotion, clone was using possible empty
collection references and has been changed to explicitly null-check instead.
POM Changes.
Spigot Changes

The set of changes which provide core Spigot functionality, which would otherwise be a mess to try and apply individually.
Merge tweaks and configuration

This allows the merging of Experience orbs, as well as the configuration of the merge radius of items. Additionally it refactors the merge algorithm to be a better experience for players.
LongHash Tweaks.

This commit adds a flat array based cache to the LongHash(Set/Map) classes leading to excellent efficiency for servers where most activity is centered around the origin (0,0)
Async Operation Catching

Catch and throw an exception when a potentially unsafe operation occurs on a thread other than the main server thread.
View Distance

This commit allows the user to select per world view distances, and view distances below 3. Be wary of the issues selecting a view distance of 1 or 2 may cause!
Address BUKKIT-3286 by firing the inventory close event when a secondary container is closed due to the player entity being destroyed. This covers all edge cases such as server stop / player kick / player quit.
Include NibbleArray from mc-dev for diff visibility
Implement 'lightening' of NibbleArrays - only allocate buffers when non-trivial value Saving from 40-45% of memory use by chunk section data.

Finish up NibbleArray lightening work - use for Snapshots, reduce copies

Fix nibble handling with NBT - arrays aren't copied by NBTByteArray
Add OldChunkLoader from mc-dev for diff visibility
Fix mcRegion-to-Anvil conversion
Update versioning check to correctly resolve api version.
Optimize packet used to unload chunks for the client

At the moment telling a client to unload a chunk involves calling the entire chunk from memory, deflating it and then sending it through the pipes even though the client ignores it and based on the bitmap simply unloads the chunk, and to add the cherry on top, this is done on the main server thread.
Skip entity.move() if we are not moving anywhere.
Alternate, sync-free-but-safe chunk reference cache
Tick loop optimization - sleep for as long as possible.
Improved Timings System

Tracks nearly every point of minecraft internals and plugin events to give a good quick overview on what is causing TPS loss.
Add oreobfuscator for Spigot.
Reduce number of LivingEntity collision checks.