[Bleeding] Implement periodic chunk garbage collector

This adds two settings to bukkit.yml, allowing activation and control of
two chunk garbage collection triggering conditions:
chunk-gc/period-in-ticks controls a periodic GC, run once every N ticks
(default is 600); chunk-gc/load-threshold causes the GC to run once
after every N calls to loadChunk() on a given world (this call is an API
call used by plugins, and is distinct from the path taken for routine
player movement-based loading).  In both cases, setting to zero will
disable the given GC scheduling strategy.

In either case, the act of doing the GC is simply one of scanning the
loaded chunks, seeing which are NOT being used by one or more players
(due to view-distance) and which are not already queued for unload, and
queueing them for a normal unload.  Ultimately, the unload is then
processed the same as if the chunk were unloaded due to leaving the
view-distance range of all players, so the impact on plugins should be
no different (and strategies such as handling the ChunkUnloadEvent in
order to prevent unload will still work).

The initial interval for the periodic GC is randomized on a per-world
basis, in order to avoid all world being GCed at the same time -
minimizing potential lag spikes.
Don't teleport entities that are considered dead. Addresses BUKKIT-1331

Teleportation should never be processed on dead entities. If you wish
to teleport an entity, do it on a living entity. If you wish to
teleport a player, set their respawn location in PlayerRespawnEvent.
/dev/null does not accept EnchantmentThorns
Return the level, not ID. Fixes BUKKIT-3326

CraftItemStack was erroneously using the enchantment ID instead of level
for the return value of remove enchantment.
Updated version to 1.4.6-R0.2 in pom.xml for Beta.
Updated version to 1.4.6-R0.3-SNAPSHOT for development towards next release.
[Bleeding] Fix corruption due to thread safety issues. Fixes BUKKIT-3333

The 'tag' NBTTagCompound field of the ItemStack assumes that it is OK to
save a reference to an NBT supplied via load() and assumes it is OK to
supply a reference to the internal field during a save(). Neither is true,
as Chunk NBT structures are required to be read-only once created (due to
being written asynchronously off the server thread AND due to the potential
to be passed to a new Chunk if the same chunk is reloaded before the
writing of the NBT is completed by the File I/O thread). Keeping a live
reference to the NBT copy passed in, or to the NBT value passed back
during saving, creates serious thread safety issues which can result in
corrupted data being written to the world data files.

The specific issue here was uncovered by the recent change to use
setName("") on the ItemStack.tag object. When a chunk is being loaded
again before its save is completed, this results in name of the field
in the NBT being set to "". This causes it to be saved as "" instead
of "tag" resulting in it not being properly reloaded in the future which
results in the itemstack losing all of its metadata.
Updated version to 1.4.6-R0.3 in pom.xml for Beta.
Updated version to 1.4.6-R0.4-SNAPSHOT for development towards next release.
Merge branch 'master' of git://github.com/Bukkit/CraftBukkit.git