Synchronize Vec3Pool to fix mod concurrency issues. Fixes #240.

Some mods access Vec3Pool in a thread-unsafe manner, causing
java.lang.NullPointerException at aok.a(Vec3Pool.java:123) crashes
on MCPC+, fairly frequently in some configurations. Synchronize the
Vec3Pool methods to fix this issue.

Thanks to nallar for this fix.
Update to Forge 7.7.1.614