Add EntityLeash and ItemLeash for diff visibility
Implement Leash API. Adds BUKKIT-4459, BUKKIT-4583
Update players when coming into view of vehicle. Fixes BUKKIT-4628

When only considering trackers from player perspective, attach entity
packet could be sent before a packet for a respective vehicle is in view
and will, in turn, be ignored.

This adds another notification when the vehicle comes into view to cover
all cases.
Fix mistranslation in isPlayerCreated() for IronGolems. Fixes BUKKIT-4543
Fix off by 1 error for spawn protection. Fixes BUKKIT-4154
POM Changes
mc-dev imports
Skeleton API Implementations
Spigot Configuration
Better Chunk Tick Selection
Crop Growth Rates
More Efficient Chunk Save Queue
More Efficient GetCubes
Proxy IP Filter
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
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!
Compressed Nibble Arrays

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
Sync Free Chunk Reference Cache