Fix floating/exploding dynamic physicals

Only put dynamics to sleep if they're at the initial location. This
prevents baskets from exploding on touch by an avatar and should
theoretically make the doggone slave cave kickables activate the pressure
plates.
Fix debug crash due to ref miscounting
Merge pull request #433 from zrax/refcnt

Fix debug crash due to ref miscounting
Fix plStringBuffer constructor with explicit size to count chars, not bytes
Merge pull request #436 from zrax/plStringBuffer_sizeof

Fix plStringBuffer constructor with explicit size to count chars, not bytes
Merge pull request #419 from Hoikas/physx_sleep

Fix long-standing physical sleep issue.
Fix dupe callbacks in VaultAddChildNode

This code called back into the python vault operation thingy, which
deletes itself. So, anytime we got dupe CBs, there was a use-after-free
error. Nasty crashes. I tightened up the CB management, threw away some
weird faux-management mess, and squashed some potential silent bugs.