Allow AsynchronousExecutor.getSkipQueue() to pull tasks from the queue
Load all already generated chunks via async chunk system

Currently we use the async chunk loading system only when players trigger
chunk loading. If a chunk is loaded for any other reason it goes through
a separate codepath. This means if a player has trigged a chunk load and
before the chunk loads something else wants the same chunk we will load it
twice and throw away the second result. With this change we instead use
the sync processing feature of the AsynchronousExecutor to load the chunk
which will pull it out of the queue if it was already queued to load. This
means we only ever load a chunk once. Because chunk generation is not
thread safe we still fallback to the vanilla chunk loading system if the
chunk does not currently exist.
[Bleeding] Handle players disconnecting during respawn. Fixes BUKKIT-4327

Prior to this commit, a player disconnected during a respawn event would
remain in memory. This causes a ghosting issue of players in the slot
count and player list, as well as a reference leak.

This commit avoids re-adding the player to the player list (and world) if
they are disconnected. This ensures that the remainder of the respawn
logic is completed as well as ensuring a duplicate player is not left on
the server.

This commit also saves the player's file at the end of the method if they
have been disconnected to ensure that their next login is accurate to the
respawn event's actions. A player that was revived and disconnected will
reconnect as revived.
Implement SpawnReason.NETHER_PORTAL and DISPENSE_EGG. Fixes BUKKIT-3148

Previously any entities spawned through dispensers (monster eggs) or
by nether portals were given the incorrect SpawnReason of SPAWNER_EGG.
This made it impossible to distinguish what exactly happened in regards
to the creature being spawned.

A method in ItemMonsterEgg has been added to further fine tune reasons
for spawning creatures. This permits the DISPENSE_EGG reason to be used
correctly and accuratly as well as the NETHER_PORTAL reason due to how
BlockPortal spawns the mobs.

The redirected method, a(World, int, double, double, double), is still
called by the ItemMonsterEgg itself and therefore uses the default reason
of SPAWNER_EGG. This does not change previous behaviour.
Update clients when ItemFrame direction is set. Fixes BUKKIT-3371

Hanging entities are placed into the entity tracker and updates are sent
out to clients for the initial placement. Thereafter data watchers are
configured to monitor the item inside the frame. However, if the
direction the ItemFrame facing changes the entity tracker will not send
out updates.

This commit removes and recreates the ItemFrame entity in the same way
that was already done for Painting entities. This causes clients to
be updated appropriately.
[Bleeding] Correctly enchant books. Fixes BUKKIT-5302

Books can now recieve more than one enchantment.  As such, breaking out of
the loop after only 1 enchantment is applied is not the correct behavior.

This commit also reworks some of the logic surrounding the application of
enchantments to the item. By checking if the event doesn't add any
enchantments rather than if the original enchantments list is empty, the
application code is only reached if enchantments are applied, rendering
the "applied" boolean no longer necessary. The ItemStack's Item should only
be set once, so it is now set outside of the loop based upon whether or not
"flag" is true (with "flag" being whether or not the ItemStack's Item is a
book).