Removing broken chunk caching from net.minecraft.server.World. Fixes BUKKIT-5425

**The Issue:**

Chunk caching in the World class is broken because it does not know about outdated cache values. This caused various problems when accessing previously unloaded chunks. BUKKIT-5425 is just one example of a bug caused by this problem, another one is the "stripes of air" appearing when using long range world modification tools (for example WorldEdit). There are probably many more problems caused by this bug.

**Justification for this PR:**

This PR addresses the issue by removing the caching. The cache did not offer significant performance increase so it can be removed.

The synchronization in the World's getChunkAt is removed, because this method may only be called by the main thread and thus it was not necessary. It is also not synchronized in vanilla Minecraft.

**PR Breakdown:**

The caching in net.minecraft.server.World is simply removed and changed back to the original NMS code.

**Testing Results and Materials:**

Here is a testing plugin (including source) and a world that will show the current bug:
http://www16.zippyshare.com/v/97128020/file.html
The World essentially contains 2 signs in adjacent chunks outside of the spawn area (thus not loaded at server start). The plugin accesses the first sign and updates the content. This causes a blockupdate that sets the cached chunk. Then the plugin accesses the second sign and tries to get the blockstate. This will cause a crash inside of CraftBukkit.
*Reproduce:* Put the world and the plugin into a CraftBukkit server, no other plugins required. Start the server. The following exception will show up (shortened):
> java.lang.NullPointerException
	at org.bukkit.craftbukkit.v1_7_R1.block.CraftSign.<init>(CraftSign.java:17) ~[craftbukkit-1.7.2-R0.4-20140216.012104-3.jar:git-Bukkit-1.7.2-R0.3-2-g85f5776-b3023jnks]
	at org.bukkit.craftbukkit.v1_7_R1.block.CraftBlock.getState(CraftBlock.java:252) ~[craftbukkit-1.7.2-R0.4-20140216.012104-3.jar:git-Bukkit-1.7.2-R0.3-2-g85f5776-b3023jnks]
	at signbug.SignPlugin.onEnable(SignPlugin.java:36) [BukkitSignBug.jar:?]

Do the same with the patched CraftBukkit. Now it works as expected.

I also tested it with WorldEdit by selecting a large plane outside of the loaded chunks and set this plane to stone. No more "air stripes" appeared. Reproducing: Use the WorldGuard plugin to select a large plane (for example 200x200) outside of the loaded chunks. Set this plane to stone. Without this patch there are stripes of air in it. With this patch it is a solid plane of stone.

A performance test plugin is available here, that updates 25600 Blocks in a single chunk for 100 times. This plugin does show, that the cache did not improve the performance, but it was slightly decreased by the unnecessary synchronization.
http://www5.zippyshare.com/v/5271139/file.html (Source included)

```
With caching/synchronization:
[01:16:45 INFO]: Block time: 15332366µs
[01:17:20 INFO]: Block time: 15238158µs
[01:17:58 INFO]: Block time: 15215652µs

Without caching/synchronization:
[01:18:32 INFO]: Block time: 10544803µs
[01:19:05 INFO]: Block time: 10398937µs
[01:19:40 INFO]: Block time: 10847860µs
```

**JIRA Ticket:**

BUKKIT-5425 - https://bukkit.atlassian.net/browse/BUKKIT-5425